Personally, I think 2003 Server Web Edition is a WONDERFUL replacement for Windows 2000. It's almost Linux-like in its speed and utility, and it plays all the games and crap that I like to have on Windows.
Anyway... forced to upgrade, a few guesses:
1. New software won't install - IIRC neither Norton AV 2004 nor 2005 will install on Windows 95. Office 2003 doesn't, either. Nor Media Player 9 or DirectX 9. Technically, you don't need any of that stuff, that's an awfully big stick for some organizations.
2. IE7-only web pages - the FEMA disaster relief forms are IE6 SP1-only... Mac and Linux, Firefox or Opera users need not apply.
3. DRM - Something you REALLY want to use requires DRM that's only supported in, say, XP and Vista. Or maybe just Vista. HDDVD, maybe.
4. Future hardware - I can forsee a day when some hardware - especially the $400 video cards we all love - won't install on a computer with a BIOS.
5. Zero driver support - Supposedly WDM works with everything from Win98 to Server 2003, but I've found WDM drivers that don't work on Win98 and/or Server 2003. Ask Tannin how much fun it is to find OS/2 drivers for modern hardware.
6. New authentication/encryption methods - Windows 2000 supports EAP, which means that things like swipe cards and fingerprint readers in theory work pretty well, but what if someone found a big, nasty flaw in 3DES or MPPE or some other encryption protocol? Windows 2000 is in "legacy support" at this point. Would they go back and add support for updated or additional security protocols to a deprecated OS?
Being "forced to upgrade" isn't really much of an issue. In reality, what will happen is that you'll eventually decide that your uber 1GB Athlon 3200 is best used as a doorstop, you'll get something else, and there will be the next version of Windows... or you'll build your own box and realize how much work it is to make Windows 2000 work on that hardware, and you'll take the path of less resistance.
Or you'll decided that you like the stupid eyecandy better than the eyecandy-less version, and that'll be the end of the discussion.