If they did that, they would have created Yet Another Alt Coin (which happens very frequently), and it would have no effect on Bitcoin at all.
Yes, I know that bitcoiners have taken solace in that belief, since the 51% risk became real. Keep believing.
There is no belief involved. What constitutes Bitcoin is defined by the consensus of the miners - period. If a majority of miners were to choose to mine according to a different protocol than what they currently do, that protocol will effectively become Bitcoin. It has nothing to do with what Gavin or the foundation want.