Just to go back to the OP for a second - why can't WOTC go back to 3.5, ask yourself this - Why didn't Paizo?
If 3.5 was still viable as a vehicle for profit, why would Paizo, a pretty savvy bunch of businesspeople, take the risk of developing a new system?
I have no idea how much Pathfinder cost to bring to market, but, it is my opinion that it can be summed up as the mathematically exacting term of <i>a lot</i>. Now, if 3.5 was still perfectly fine, as is being suggested in the OP, then why bother? Why spend all that money on something that very well might not work when there is a perfectly viable option right there to be used?
After all, 3.5 is OGL, and it's already been shown that you can repackage [ame=http://www.amazon.com/Mongoose-Pocket-Players-Handbook/dp/1904577660] the Players Handbook[/ame] and sell it. Why wouldn't Paizo save themselves a WHOLE lot of hassle, reissue the Core 3 with new art and new flavour text and be done with it?
Unless, of course, 3.5 was largely played out and there truly did need to be something new in order to get people buying again.
So, in answer to the question, "What's stopping WOTC from going back to 3.5?", I'd simply point to Paizo and say, "Well, if it was a viable option, why didn't they do it?"