Generally agree about the time lock tied in with the beacon.
However.
They have 70 bajillion souped up Star Destroyers hanging out there.
You destroy the beacon…um…they build another one. Or two. Just to be safe.
Unless they’re out of building resouces from crafting 70 bajillion souped up Star Destroyers.
That’s a plot hole written by JJ Abrams and Chris Terrio… luckily Palpatine explodes or something and all the ships blow up too? It’s kinda unclear… They do blow up a few by destroying the canons, but the majority just… ¯_(ツ)_/¯