Edit: but, their main codebase is in Flow, and that sounds like a mess to migrate, so I wouldn't be _too_ worried. It might slow down, but I doubt it will become unmaintained anytime before Facebook gets sued out of business :-P
I think Flow's in more direct trouble, but I wouldn't announce victory for TypeScript just yet. With wasm around the corner, things may (or may not, shrug) change significantly.
Even with wasm, much as I'm looking forward to the Great JS Purge personally, it's not going to happen anytime soon - too much existing code and devices. JS will remain necessary on the front-end for at least another decade, and probably beyond that. So TS will still be necessary to make the pill less bitter.
Edit: but, their main codebase is in Flow, and that sounds like a mess to migrate, so I wouldn't be _too_ worried. It might slow down, but I doubt it will become unmaintained anytime before Facebook gets sued out of business :-P