The problem with the wholesale deletion of comments is that it also affects other people. For example if we have a back-and-forth constructive conversation here and one of us deletes all comments, then the value of the other person's comments are diminished, and sometimes even incomprehensible.
It's pretty clear you're putting something in the public when you're commenting on HN; this isn't a surprise and nothing is done surreptitiously. If you contribute to a debate in some TV discussion programme then you can't have that deleted later either.
And there are options without wholesale deletion: specific comments can be deleted or edited for specific reasons, and your account can be "soft-deleted" by changing your username to something random.
If you want to have more ephemeral temporary conversations then that's fair! But HN is not the right platform for that, IMHO.
Yes, I do see the irony of writing that here. :'(