Even if they do, they would not be able to force instance owners to update to the closed version. And people would take over the last available open source version and fork it. Also, a closed and open source version could co-exist, since the api is open.

If they went closed source that would mean they would have a similar situation to Emby. They went closed source after a time which gave rise to people forking the last public build and making Jellyfin which is an excellent alternative.

Protocol is open, anyone can rewrite a federation client that pulls data off proprietary servers as a last resort. This is why federation is great. Besides, you can't close an open source project, you have to create new, closed parts to have them. That's how mariadb and galera took a chunk off mysql user base and how libreoffice became a successor of openoffice.

As a side note, I wish we could move accounts and even communities between instances, based on some kind of two way handshake agreement.