While I think 3.5 players are more likely to stick with 3.5 if 4e sucks, than to switch to earlier editions, its not implausible that a few may go back to older systems. I myself have had an urge now and then to play a B/X game, but I'd never go back to 1e (MPO). The kind of simplification 4e seems to be aiming for is not the same as the simplification found in earlier systems.
rant/
Though I really roll my eyes when I see all the vitrol being spewed about this edition, vs. that edition, etc.
Its a game. Please chill.
The preceeding comes from an ubergeek who plays in/runs and average of two games a week. Yes, I need a life, I know. But if I don't get all bent out of shape if someone doesn't like "my" game edition, I can't fathom why everyone else does. However, I do start to get annoyed when I see thread after thread of arguments about it. Our hobby is far to niche to be gettin' all upidy about what system could "beat up" whatever other system. This is why I generally shy away from discussing different systems on this site, or any other. It gets to feel like you just told someone that their kid was the most godawful ugly spawn in existence if you express a negative about their pet edition, even if you gave several things you liked about it in the same paragraph.
/rant