You are correct, and I’d like to also take a moment to point out a timeline error I’ve made thus far.
Sylvanas’s blitz had already started before the Cataclsym; she had forces trying to take out the wall before it fell due to Deathwing.
So, just to keep it all clear: Rebellion, worgen gets summoned, worgen start spreading but it’s kept under wraps, underwraps stops working, some night elves arrive, curse gets under control, Sylvanas is at the gates, Deathwing Happens, Sylvanas runs roughshod over Gilneas right after.
With all that said.
But… Why would they?
By the time Sylvanas is even considering making her move, the worgen situation is largely under control, thanks to the night elves. Some specialists might be brought in to help develop a full cure (which clearly wouldn’t have worked, because there is still no full cure in the post-Cata world, which has gone on for over a decade in-universe), but the threat itself is gone.
Then Sylvanas blitzes.
We know she’s moving quick; the entire narrative shows this is a rolling machine of death that isn’t pausing, because it doesn’t need to. While the Gilneans would now be able to call for reinforcements from Stormwind and Ironforge, they don’t have a whole wall to stop Sylvanas from just… Going straight on into their lands. There’s no pause at all.
Then Deathwing does a Deathwing.
The forsaken army was right outside when Deathwing Deathwinged, and were relatively unphased. Presumably being within the actual Gilnean kingdom and actively fighting would have hindered them, but it doesn’t seem likely by any meaningful measure; if it didn’t outside, why would it inside?
Sure, some extra ships would have arrived to evacuate more Gilneans, but would there have been time to muster up enough forces to actually push Sylvanas back? Wouldn’t there have been even less time, given there isn’t even a pause in her strike?
I don’t see this affecting the outcome in a meaningful way.
However…
If Gilneas’ isolation is supposed to be depicted as horrible, then what would have stopped the Scourge? We know because Bradenwood exists that the Scourge threatened right up to Pyrewood, just outside the gates. We know the Scourge actually made it to the gates and were stopped. What would have spared a non-isolated Gilneas from that?
The wall was only built because Genn wanted to isolate his people. If that choice is so bad, how is the alternative scenario better? How is isolation the cause of his problems if the alternative is Gilnean Forsaken at best and Gilnean Scourge at worst?
No, this is why I don’t by the idea that the Gilneas storyline is about how isolation was the problem. Isolation avoided the worst outcome. Isolation worsened the alternative, the events that did happen. But the fall of Gilneas had little to do with isolation.
The Fall of Gilneas happened because nobody expected Sylvanas to go full Banshee Queen, or to go as hard at it as she did.
The lesson of Gilneas isn’t about isolation.
The lesson of Gilneas is to not give up.
Not when the wolves are at your door, not when the world shatters, and not when the zombie apocalypse you hoped to avoid finally walks down your street. That’s why the continuation of the Gilneas story (which Alliance players don’t even get to play through, big brain move there) has Gilneans still fighting.