Personally, I like to think that Spearbreakers didn't get abandoned at all. What I imagined was that, after the Battle for Spearbreakers, pretty much every major power on the planet had its strength broken. The spawn were temporarily pushed back, Parasol and Ballpoint could no longer justify their continued presence and took everything they owned that wasn't bolted down or breathing and the necromancers' armies were wiped of the face of the planet. The only military power left was Spearbreakers, which was not at all equipped to handle the number of refugees that sought shelter there. Spearbreakers expanded into a refugee city which then expanded into a real city. Eventually Spearbreakers became the new de facto capital seeing as it was the only city left standing.
I like the idea of leaving Spearbreakers alive, in the spirit of the new update and being able to retire fortresses.
I've been watching Kerbalkon and just heard "Landing is just crashing slow enough that you don't break anything."
The problem with alot of that being quite simply that The Battle of Spearbreakers was just that. It wasn't a continent spanning battle unless megaportals are taken into consideration, meaning following said fight, the fortress was still just a fortress like any other. The old capital was still standing (more or less anyway, based on what was dug out of legends mode,) The Copper Realm was still pissed off at it and siccing mercs on the place left and right, and the spawn simply kept attacking everything, their numbers only just kept in check by the constant fighting.
And as all fortresses must, either internal or external forces would have eventually made it cave in on itself like a neutron star, but probably not until time did the one thing that hordes of spawn biting and clawing at Fischer's head couldn't. I mean sure, there would have been other champions to follow, but I doubt they'd be able to hold a candle to her badassery. And even the fort's best can only do so much against overwhelming numbers hitting them with concentrated force all at once, and said concentrated force would have still probably gotten its shit slapped in the process. And even through all the horror and the now certain decline, the populous managed to avoid a tantrum spiral that would have surely left no survivors.
And of course there's still the fact that unlike many places, the fort did survive in spirit with those who survived packing up and moving on (I like to think it was probably 20%~ that survived and spread the stories that grew to the legends many will know by the time of SBII.) Who knows? Perhaps SBII will be founding by those looking to surpass the might of their ancestors in the old legends...