I have several reasons for not replacing my trusty V3 with a new card.
1. If it's not broke, don't fix it. I got my Voodoo 3 specifically for use with D3 and FS2. It's worked great for a long while in my old computer (a non-MMX Pentium 1 166 Mhz with 32MB RAM) and now in my new 933 Mhz computer.
2. I never said it couldn't do 1024x768, it can, I'd just never installed the high-res pack. I just proved it to myself. I ran both the first regular mission in the FS2 campaign and my mission on my Robotech MODed FS2, which was a bit jumpy at one point but considering the mod has fighters with 1000+ polys and 1700+ poly scout ships, and it would have had ships with a combined total poly count of about 14,000 on the screen at one time, that wasn't surprising. Gameplay was smoother once I got into combat. AOL being running might have affected that too. Oh, and all those ships don't have multiple LODs. (Making sure the uninstaller wouldn't screw anything up when I uninstalled my formerly "clean" copy wasn't fun, FS2 wasn't designed to be installed twice on a computer.)
3. Glide. I chose the Voodoo3 partly because it supports D3D, OpenGL, and especially Glide. Unless I replaced it with a V4/5 I'd have to give up Glide and programs I have which are Glide only.
4. Money and priorities. I need a new sound card more then a new video card and smaller speakers more then I need a new sound card. The list goes on.
5. Resolution wise, I won't want to use 1024x768 or higher much because of the simple fact that I have a 15" monitor. Higher resolutions then 800x600 just look too small. There's also the fact that FS2's resolution changing screws up my desktop icons if it's set for 1024x768. (It starts off in 1024x768, switches to 640x480 for the intro movie, switches to 800x600 briefly, then back to 1024x768 for the game.)
6. The main campaign I've been interested in, the Robotech MOD, doesn't have 1024x768 artwork and it doesn't look right without it.