If their side don't reach tier 5, then they stay in the alliance, even if our side don't reach tier 5. And they aren't going to reach tier 5; because, they have no incentive to do so.
If they reach tier 5, then a bunch of systems leave the alliance. That already happens quite often, anyway. So, it doesn't give them a new storyline. So, why put the effort in?
Now, here's what I think should happen, in order to address that situation. If they reach tier 5 before we do, then our side immediately become personae non-grata in Reorte. The megaship that we are all docked at has no choice other than to jump away without delay, and we all end up stranded in deep space, somewhere very close to the system that contains Raxxla.