I pretty much haven't stopped thinking about this for the last sixteen hours
You know what I just realized? That by stranding a partially-human version of the Doctor in Rose's parallel world, they have in effect ensured that they can bring back David Tennant whenever they want.
So, in effect, David Tennant gets what I imagine he wants - the possibility of continuing to play the Doctor on occasion without having to commit his career to it.
Huh. I'm not sure how I feel about that - I mean, I love David Tennant's Doctor, but there's such a thing as too much hedging of your bets. I suppose it will depend on if or how this ever happens. Thirty years from now, the Doctor and Rose returning ala Sarah Jane might be kind of cool? (And shut up, you know they're going to milk that cash cow for all it's worth.)