Personally, I'm not convinced that they'll turn off 4ed DDi. If anything, I think it might just end up in a "frozen" state. i.e. "We'll keep the servers up and running but don't expect any changes to the actual layout, etc." From my early looks at the VTT (haven't used it in a long time) it certainly seemed that they were contemplating making it available for multiple editions.
All that being said though, assuming for argument's sake that they do turn off the DDi, I don't know whether I'd run another game or not. Playing as a PC wouldn't be a problem and I would certainly be willing to do that. Whether or not I chose to run a game would depend largely upon a) how much I like D&DNext, b) what other games I had players for, and c) how easy D&DNext is to run.
So far, I've seen things I like and things I don't like with D&DNext. Its simply too early to call it since we've actually been shown very little. I like the idea of faster combats, but some of my players I know won't. I have at least a couple of players who pretty much tune out the moment combat ends (which is fine, its just another playstyle after all). So I have a feeling that in many respects, those players might prefer 4ed over Next since they are not into the exploration and roleplay pillars.
I do have to admit though that when my current campaign ends (currently at 18th level, hoping to hit 30) I'll probably be ready for a break regardless. We've been playing close to 3 years now and I anticipate it will likely take another 18 months or so to finish out (maybe a bit more). So yeah, odds are that I'll want to take a break anyway and will almost certainly check out what else there is out there. That said though, it will be hard to pull me away from D&D for too long.