I ask this because the Steam release of D3 is just a straight up rerelease. Nothing has been done to fix certain problems... like alt-tabbing and getting an exception error, which you can't actually acknowledge and have to close the game from the task manager. Has anything actually been done to the game besides erasing options from the launcher menu?

I don't believe so, although I'm not 100% sure.

They don't have the rights to edit the code. That belongs to Interplay or Outrage.

D3 works fine without dosbox as it was a Win98/xp game. I have the original CD and install it without any issues.

More specifically, they probably don't *have* the code. Descent 3's source code was never released and it's unlikely that GOG was given it. If they had, you'd likely see patches above and beyond what had been originally released and the Descent community (outside here) would be aware of its existence. No evidence of a thing is just that.