If they go close source, other people will take the last version of the code and build on it

The original creators can sure try, but since Lemmy is ACTUALLY open source, the community can just fork the source, call it "the-good-lemmy" or whatever, and devote our time & resources to it instead of using the bullshit version

No. The API debacle was fundamentally about money, after all. In the very unlikely event someone does something similarly one-sided and stupid with a fediverse offering, people will simply fork it or move to different ActivityPub compliant software. Neither is possible on Reddit, a proprietary, for-profit website.

Our values are completely different from big tech. We would never do this.

As other people mentioned, it'd be impossible even if we wanted to, because people would likely fork the code.

decentralisation

I guess I don't see what the incentive would be for this, or even what it realistically means in this case.

Do you mean like relicensing the backend and frontend with a closed source license? I don't see what the incentive would be for that unless they wanted lemmyml to be the only instance in existence (which runs counter to it's raison d'etre) and to make secret/proprietary/commercial extensions to it that are difficult to develop in the open.

Or I guess unless they wanted to start charging instance admins for the honor and pleasure of running their software, which at least right now would be the quickest way to ensure nobody runs their software.

This was asked before, but it is under the AGPL (which means that if you modify the code you must make the modifications public), to make it a closed source project you would have to get the agreement of every contributor or rewrite it's code which is very hard to do (and i don't think i ever heard something like this happened). The federated aspect is another line of defense.