They can still threaten to sue things that cross the line or come close and make threats about their claimed IP as TSR did, which will suppress even more the small number who go without a safe harbor.
Not without huge reputational risks.
It's amazing to me just how many of you people are stuck in 1990 when it comes to the way the world works.
What, EXACTLY do you think is going to happen, when WotC sends a threatening letter to some small-but-popular 3PP? Seriously what do YOU think is going to happen?
In 1990, yeah, sure that would have worked well, there was no way to publicise it, and no-one would have cared anyway, and even if they did, they couldn't have helped.
In 2023, on the other hand, that C&D will IMMEDIATELY end up on the internet, and literally hundreds of people will be trying to make clicks off it. And none of those clicks will be in WotC's favour. People will demand WotC answer as to why they did this, and this whole outrage cycle we're having right now? It'll kick off again. People who've gone back to Beyond will start cancelling accounts again. People will stop buying WotC products for a while. WotC's reputation will get even lower. Eventually they're going to end up as the "EA of the TTRPG industry" if they do stuff like this.