I didn’t delete the comment thread, but I’ve read the deleted comments. I don’t see any that should be restored because they add value to the question. The interesting back-and-forth was mostly on a tangentially related topic. I understand the purpose of the move-to-chat feature as a way to support an extended discussion about the question, not as a way to preserve every interesting conversation that happens. If there’s something in an SE comment thread that you would like to be preserved permanently, you should take action to preserve it: move it into a question, an answer, or start a chat room yourself. To some extent, the more accommodating the moderators are about preserving inappropriate uses of comments, the more work we make for ourselves from users who don’t use comments for their intended purposes.
I’d like to address two errors in this sentence of yours:
This is insane, there were valuable comments. They should have been moved to chat if they were too numerous as this is usually done, not deleted!
The first is an etiquette error. “Insane” is not a synonym for “undesirable,” “upsetting,” or “confusing,” and using it that way is unwelcoming for folks who deal with real mental health issues. That usage is common; let’s make it less common here in our little corner of the internet.
The second is the assumption that comments are “usually” moved to chat. There is a selection bias there, because moving the comments to chat generates an informative comment, while simply removing problematic comments does not. I think it was the case when I started as a moderator that more deleted comments were moved to chat in big batches than removed singly; however, in the years since, our community has evolved away from chatty comment threads, so I’m not sure whether that’s still the case or not.