I didn't mean that they canned Baker because the show wasn't being watched, I meant they regenerated rather than cancelling due to ratings. The first Doctor couldn't play the part anymore, so instead of cancelling the show (because of viewers), they invented regeneration so they could recast it and have it be legit.
Every other time the producers thought the actor was no longer right or if the actor himself no longer wanted to play the role, they weighed the options. "Should we end the show or not?" 11 of those 12 times they decided to keep the show going by recasting.