comments often respond to other comments. if the latter are deleted, the former make no sense. my view is that once you've said it, you should be prepared to take the consequences. erasure of comments is an attempt to rewrite history. not so good. is it possible to disable comment erasure?
I don't think this is a good idea.
Originally, comments were meant for short-term discussion in order to improve the post. They should not contain anything valuable (if it is valuable, it should be incorporated into the post), and can hence be deleted. Since then, SE has partially drifted away from this core idea, but it explains why comments are deletable.
Still, something similar applies here. If the discussion is valuable, then the gist can be incorporated into the post, and your comments can be deleted as well. If it wasn't valuable, then your comments can be deleted anyway and no harm is done.
In principle users should be able to take back what they wrote. The real-life way of taking something back (saying that you do) would be inconvenient here, since many people read only parts of discussions and may then think you have a view that you abolished later on. If people weren't able to retract their comments, I suspect many would request that their comments are anonymised (which is the right of the users here), and that would just mean a lot of work.
We are not here to win discussions, but to support one another in their study of philosophy.