Yeah, if I had to hazard a guess, it would be that the folks still running a 32 bit O/S, probably don't have the hardware to run FO:4 in any event...... I still see them from time to time at work, mostly Vista, and an occasional Win7 32 bit version, but, it's all becoming rather rare. And yes, XP is still out there, but, I suspect the hardware it is running on wouldn't have a hope in hell of running FO:4...... or even Skyrim...... at least, not at playable framerates. The world has finally caught up.
The game engine has indeed evolved. With each iteration, it has new capabilities, some old bugs get fixed, some new bugs get introduced. I suppose expecting anything else, would be bordering on wishful thinking. Lets just hope the game has evolved in better shape than Johnny Cash's http://www.streetlegaltv.com/news/case-of-the-mondays-johnny-cashs-one-piece-at-a-time-cadillac/.....