They'll need to be careful. Both BlueSky and X may launch covert, bot-powered attacks to try to take down or cripple the largest Mastodon instances. I am also concerned about GitHub (many projects are run by very few devs who aren't invincible and can get scammed into accidentally including malware in an official update), and Lemmy, similarly, and now Android's closing ecosystem.
The Web is becoming so nuts lately... Capitalism is truly cancerous and ruthless. I don't know what solution there is to this madness... maybe just Linux-everything and a return to local intranets instead of the Internet, but that has its own numerous challenges...