All answers should be editable so that they can be improved over time. The more highly upvoted an answer is, the more strongly you should consider your actions before making any changes, but even a very good answer that 10 people have found helpful still stands to grow and improve over time.
As for deleting answers and comments - I mostly agree with you. Comments should not be deleted unless they are spam or offensive. Answers should generally not be removed if they have upvotes, unless they can be merged with an already existing answer with more up votes that contains all of the same information and more.
