Unless I'm remembering wrong, Mojang can't adopt the Forge code because I think it's under GPL (or similar). The problem being that it requires any project using it's code to be released under the same license (it's kinda the licensing equivalent of a virus), which is great, but the open-source/free thing doesn't seem to fit with Mojang's methods (and they're unlikely to shift that just to acquire forge).
Also, I'd disagree that Mods are the "future" of minecraft. To my eye, mods change the ethos of the game. Mojang's version is intended as very simplistic, basic tools, with complex ends. The redstone system is a prime example of this. They lay down some simple rules, and the fun is what you can do with it. The mods, on the other hand switch that to complex, powerful tools, many of which are the target themselves (making a T5 spawner, getting a gravity suit, etc.), rather than the vanilla goals, which tend to spring from the user.
I would love to see a decent api, but it'll likely end up being a rebuilt version of Forge, done in collaboration over an extended period. Given how far forge has come over the past year even, it's easy to see that it's not going to be a trivial matter.
