That already happened. It's just that some people in the community, some being the "XM team" and others in this very thread, have selfishly sat on private builds of ProSystem for years with fixes and improvements while the state of 7800 emulation stagnated:
1) http://atariage.com/forums/topic/160363-prosystem-developer-requests/
2) http://atariage.com/forums/topic/167421-super-circus-atariage-new-wip/page-7?do=findComment&comment=2115736
3) http://atariage.com/forums/topic/213129-trouble-using-16-bit-pokey-sound-on-atari-7800/
4) http://atariage.com/forums/topic/214554-pro-system-emulator-windows-8/?do=findComment&comment=2793222
A really awful thing to do considering that ProSystem was originally open source for the benefit of everybody. Just imagine if a project such as MAME/MESS kept improvements to open source code private amongst themselves. I reached out to one of those with the "private" ProSystem builds requesting source code and was ignored. Today, you'll get a lot of hand waving from them telling you to just use MAME/MESS now that its 7800/TIA/MARIA/POKEY modules have been improved for accuracy, as if they are now off the hook for the rotten thing they did.
Some time ago I ported ProSystem to Libretro API (https://github.com/libretro/prosystem-libretro) so that it's playble in their cross-platform frontend, RetroArch, though still missing the improvements from Wii7800. Raz0red has his old Wii ports on GitHub now and I actually sent a patch his way for posterity's sake to fix loading of good dumps of 64K games: https://github.com/raz0red/wii7800
Anyway, my own port of ProSystem for OpenEmu on OS X (https://github.com/OpenEmu/ProSystem-Core) is being updated with the fixes from Wii7800. Then somebody may want to port those over to the other open source forks if they still find ProSystem usable.