Well, we have yet to find out how vast a departure 5E will be from 4E. Keep in mind how little time elapsed between 3.0 and 3.5 (and while the core gameplay is largely the same, the tweaks and outright changes make the two versions notably different).
Also, this is just an announcement- It's not being released now. 3.5 was released in 2003, and 4E was announced in 2007 (about the same amount of time as this). While I'm not trying to stick up for them, 4E has not been nearly as successful as WotC was hoping, and from what I've read, sales have been in decline with other systems picking up steam in their stead. It makes sense for them to begin developing the next phase, and at least they seem to be trying to figure out what went wrong and what went right, and are trying to involve the fanbase in development at least more than they did before. I don't necessarily think it'll work out, but it seems like an honorable enough intention.
I happen to be a fan of both 3.5 and 4E, so I'll likely continue playing both even as 5E starts releasing. I am, however, eager to see what changes the revision will bring, and how fun the gameplay might or might not be.