The great thing about the OGL is that it's permanent. WotC could go out of business tomorrow, and the 3rd party publishers could keep publishing until the market dried up and blew away like so much dust from a demilich. The thing about the OGL is that it can't "go bye bye." Nobody can shut it down now that the cat's out of the bag. Once something is declared open content, it's open forever. If 4E turns out to be a closed system (and IMO they'd be cutting their own throats to do that), 3.5 will still be open.
As for people dropping 3.5 for a non-OGL 4.0, I think that there's enough 3rd-party support to prevent the same kind of mass migration as we saw from 2nd edition. Remember, when 3rd edition came out, D&D was pretty much dead. TSR had gone under, and there was no support for the old material. The new material was a leap forward and came with tons of support, and still does. 4.0 will have to reinvent the wheel in order to be a compelling enough change to pull people away from what is essentially a tried-and-true system. And even if WotC dumps 3.5 completely, there's still the same bunch of companies providing 3rd party support. Probably more, since the core publisher will be out of the picture.
If 4.0 isn't open content, I predict a schism. I don't anticipate wanting to move to a new edition. It would have to be solid gold. I figure many people feel the same way. Especially if it means having to leave behind Green Ronin, Malhavoc, AEG, and the rest of the publishers we depend on for good support products. We've been through a monopoly era before. And now we've seen what happens with an open system. The open system makes a better market and provides us with better products. If you want people to play a new system, you have to give them a reason to do so. And that reason has to be: it's better than playing the old system. If you can't provide that, your product will fail.