I do recall a comment/thread saying how HN comments/behaviour was starting to resemble Reddit, so there may be something to that (though I'd chalk it up to becoming more popular/mainstream).
I would mention /r/credibledefense as a counter example. It was a niche subreddit before the Russian invasion of Ukraine, in the aftermath got popular, but still managed to keep its quality.
But I agree with your general point, it's not a huge problem for me, though. Some subreddits get popular and thus bad, some new subreddits get created. Reddit's relative anarchy is its saving grace, communities can split/fork, but still stay within reddit platform.
Format certainly influences the quality of interactions, but there are some other strong factors as well.