Half the fun of writing science fiction can be coming up with novel concepts or 'rules', what happens when everything of consequence in the 'Jamverse' has been built out? Will they start a new one?

Shared fiction communities happen naturally when you have simple rules and a strong appealing grounding (visually, intellectually, ritually, or otherwise).

See: Minecraft, Sonic, MLP, SCP, even Bored Apes Yacht Club for 15 minutes

Which is me agreeing with you: this seems too hard to get; there's already 4 timelines and no faces to get attached to.

So why wouldn't I just write my own stories unrelated to this?

I agree that there's a tension you're pointing to here. But I can imagine narrative devices to get around it, if it's ever a problem.

Maybe go one level deeper? Or maybe branching the timeline in the past, like Marvel popularized?