Hah. No.
Microsoft may have claimed now that they're going to support the 360 for a long time, but Microsoft has lied about support longevity before and they will lie about it again. The moment the 360 stops making enough money to justify Microsoft holding off on releasing a new console to the market, they'll deny the 360 ever existed and press on.
Microsoft is likely to play a very careful game of cards here; it's clear that they're waiting for Sony and Nintendo's next move. They do not have the kind of lead on the market that they wanted. Microsoft is the kind of company that wants 90% market share in everything they do -- they thought beating Sony and Nintendo to this generation would do it for them and it didn't. They won't repeat that move, but they're also not going to wait. The 360 does not have the kind of horsepower to compete in multiple generations.