Fediverse right now is mostly used by a small group of enthusiasts.
If Meta joins the fediverse, the people who join Lemmy/Kbin/Mastodon instances will still be the same group of enthusiasts.
If Meta leaves the fediverse, you will also still have the same group of enthusiasts, that you have here right now.
I don't get why the EEE meme is so prevalent here. XMPP never died and big tech never even harmed it.
Best case: Big corps joining the fediverse could make the ActivityPub the de facto standard for a new generation of social media. Similar to how HTTP, Email or the Linux kernel is practically used everywhere now.
Worst case: The fediverse will remain as obscure as it is right now and nothing changes.