Moving from Mac to PC in a prepress environment isn't something to be taken lightly. I've been in digital prepress for almost 30 years, and I currently work as a CTO for a $7M printer, and am fluent in both IT and prepress. I'm a die-hard Mac guy, but I do almost all the Windows support myself. Over the years, there are many conclusions I can draw from personal experience supporting both. I have a mixed environment with 6 Mac stations, 3 Mac Servers, 6 Windows Servers, and about 30 Windows desktops and laptops. If I attempted to switch to Windows-based prepress, here are things I would expect to see:
1. Windows PC's have a much shorter service life. Under Windows XP in a typical office environment, I begin to see significant performance problems after about 2 years. By the 3rd year, the problems are basically fatal, and requires a complete overhaul and reinstallation of Windows. If successful, I can see maybe another year of service life in a lower-level capacity. Effectively, functional replacement happens around 2 years for 50% of my Windows stations. At 3 years, it's closer to 75% non-functional, and mechanical replacement rate is 25% at this age. By year 4, I've turned over almost 100% of the Windows hardware for newer computers. By comparison, all 6 of my Mac stations are on year 5 and have not required a re-installed OS. In fact, some are carrying migration data and apps that are 8+ years old (Apple's transfer program is wonderful). I use several Windows XP systems in prepress, and they are all but unusable after 3 years as well. Apple, quite frankly, builds a mechanically solid machine in the Mac Pro series. Great ventilation is one reason, but the hardware just lasts longer too. I bought a lot of LCD monitors from Acer and Asus over the years. The Acer's are a fraction of the cost of Apple's LCD panels, but have 90% failure rate at 3 years (over 50+ units). My only two Apple LCDs are still running after 5 years, but they were 4X the cost of the Acers too. I've switched to Asus monitors, but the oldest of them are 2 years old and have not have any failures yet.
2. Windows PC's have drastically higher maintenance costs. While I can't put it into exact dollars, I can give you pretty accurate ratios based on my time. Even corrected for my user base, maintenance of a Windows XP machine is 15-18x of the prepress Macs. By "maintenance", I refer to updates, patches, user assistance with performance issues, virus, spyware, and protection, mostly related to the Windows XP operating system itself. Every single Windows machine I have has fallen to malicious software at some point (except my Windows Servers). This is more of a user issue, as they are reasonably protected by Symantec and Malwarebytes. A determined user can still easily infect their machines. The prepress Windows machines have faired better, but still have at least 10X the maintenance of the Macs. I spend about 20-30 hours per year fighting Windows viruses and malware. On the Mac, I spent zero hours in the last 10 years.
3. Uptime ratios heavily favor the Macs. Performance of the Macs compared to when new is virtually undiminished, while Windows performance degraded considerably over time (I'd say 20% per year is reasonable). Productivity is difficult to measure, but in an office environment, the difference in downtime between the two platforms is ridiculous. For me, an average office and prepress PC has between 5-10 hours of maintenance in the first year, rising about 50% per year afterwards. The Macs average less than one hour, some are close to zero. Stability: the Macs average 30-45 days between restarts, and the Mac servers probably average 6 months between patch-required restarts. Nothing running Windows last longer than a couple weeks before a restart, even the servers.
4. Prepress Mac workstation costs the same, if not less than similar workstation hardware for Windows. While office-grade Windows machines are dirt cheap, the Mac's are far more price-competitive on the higher end of the computing scale. A typical office-grade Windows 7 desktop for me is about $450 (including keyboard mouse) plus monitor. The cheapest Mac Mini I can get is $600 (lower hardware specs, and I need a keyboard/mouse), and Office costs more for Mac too. That is just one part of the TCO though.
On the upper end, a Mac Pro tower with 2.66Ghz Dual Quad-core (8-way) Neleham, 6Gb RAM, 640Gb drive, NVIDIA GT 120, etc runs about $4700. The Dell Precision T7500 Workstation with the same CPU, 4Gb RAM (max usable for Win 7 Pro, according to Dell?), 500Gb drive, 512MB NVIDIA Quadro FX 580, etc runs $4,900. It's hard to get an Apples-to-apples comparison, but if you are honest, you will find that Apple's high-end hardware is pretty reasonable, if not a bargain. Granted, you can skew the Dell's with lower end chassis for a lot of savings, but we know that Apple's well-engineered tower competes with the upper end of Dell's workstation line (Precisions). The Mac Pro is a nicely finished hunk of machined aluminum, and you really can't put that against a sheet-metal-and-plastic Dimension tower for heat-dissipation, airflow, and noise.
5. Multi-monitor setups are superior on the Mac than PC. Under XP, setting up multiple monitors required more software and is much fussier than the Mac's built-in ability. In all honestly, that may not apply under Windows 7, which I only have deployed in office environments (I'm not using Win 7 in multi-monitor prepress yet).
Some other factors to consider too: we take in 70% of our production output as customer generated files. The remaining 30% is generated in-house by staff designers. For incoming files, 95%+ are Mac-based, and most require Mac-based fonts and most lack image extensions. Quite frankly, we are driven by what the customer gives us. If they, overnight, decided to switch completely to Windows and MS Publisher, we'd buy a bunch of Windows systems and copies of MS Publisher. Whatever they run, we run. Right now, they run Macs, and so do we.
For that other 30% of content we create and design in-house, we have a choice of any app/platform in the building. After years of doing this, the Mac is simply faster and cheaper to get from idea to paper. Much of that is user comfort, but a lot of it is inherently in the productivity in the prepress environment on Macs, even though the software is the same. Trust me, if we could generate a better quality product in less time with Windows, I'd make the designers use it. Fact is, Windows isn't better, partially because of the maintenance and uptime ratios I mentioned earlier. Don't take my word for it: ad agencies and marketing departments know this too. This isn't just resistance to change by the designers either: look at how fast they switched to InDesign from Quark. Given something that is better, faster, or more transparent from design to paper, the creatives will adopt it en-mass, and practically overnight. Creative types are kinda lazy that way. Show me an enterprise full of Macs instead of Windows, and I will show you a lazy IT Director. Nothing wrong with that, in my opinion. IT and MIS are not value-added services. Prepress is.
One last thing I will emphasis, and it's the key to the enterprise Windows/Mac issue. IT departments want to stay IT departments. They want to keep their jobs, or even increase their staff if possible. Large departments become more powerful in the company, command higher budgets, and IT people hold tremendous job security. Windows, with it's high maintenance costs and short life cycle, represent constant, guaranteed work (in any given year, I replace at least 5 Windows machines). Deploying Macs over the enterprise would likely decrease TCO by a factor of 2X or more, while the original post's replacement of 90 Macs in the prepress department would likely result in 1-2 more IT hires. Either way, prepress department productivity goes down under Windows.
To the original poster, that's probably the most truthful answer to "Why?"
It's job security for the people who want you to switch.
If that sounds self-serving, its likely because it is.
Final Disclaimer: my experiences may be different from yours. Someone better skilled in Windows and less adept on Macs may find their time spent differently. Still, I wouldn't expect the TCO to tip towards moving to Windows-based prepress. Your mileage may vary.