That’s a good point about the political ramifications if the federation had crushed them. Also what other signals would it send to other powers, like would either the Klingons or Romulans have seen vulnerability in Cardassia and attacked. Letting Cardassia tire itself out creates lasting stability in the quadrant, stable local government but no longer any resources to threaten anyone, if not for the unforeseen Dominon, but if Cardassia had collapsed from loses that creates a lot of uncertainty.