One obvious problem with it is that if Twitter dies, some other site will take its place. The idea of tweeting isn’t going anywhere. It’d be a bit like trying to uninvent a bicycle.
More generally, a bunch of thoughtful people use Twitter, and Twitter DMs changed my life. It’s the AOL messenger of the 2020s. A low friction “DMs open” platform is very hard to come by — the closest before the messenger era was email, which usually isn’t a conversation. So there would be a real loss in terms of social value. E.g. TikTok requires both people follow each other before DMing, so there’s not even an option.
You’re not entirely mistaken, but the caveats seem worth calling out. On the whole it seems like more harm than good would come from the implosion.