Sherlock, I think the way the nav-tower works is that they didn’t blow it up, they blew up the power source for it, which delayed the ships taking off. Though it’s not at all clear in the movie, I’m pretty sure that was their plan all along - to just stall the fleet long enough for Lando & Co to arrive (because if deactivating the tower is enough to stop the fleet then why do they need Lando’s help at all?). It isn’t until Finn & Jannah destroy the command deck that the fleet gets stuck there for good.
So…the First/Final Order just needs one of its millions of personnel to find a battery, and they’ll be able to recharge the tower and be on their way? Does the tower on the ground plug into the same power source as the tower on the space ship? And why would destroying one ship (the command ship) cause the rest of the fleet to be stuck (unless it’s in charge of their navigation signal)? The command ship wasn’t the Resistance’s target - nor was it of any importance - until it assumed navigational control. And they didn’t even know for sure that anyone would show up, so their plan wouldn’t have deliberately hinged on whether or not they got reinforcements.
This is all kind of expanding on the problem I pointed out - this Nav Tower idea is so messy that it’s hard to follow both the goals and the stakes of the battle. If we streamline it to “Take out the navigation tower on that command ship and the fleet will be stuck,” we avoid all of this confusion.