From Ernest Hemingway’s
The Sun Also Rises:
“How did you go bankrupt?” Bill asked.
“Two ways,” Mike said. “Gradually, then suddenly.”
Societies don't get destroyed by a catasclimic event. Well... these do happen, like the fall of the roman empire had their share of it... But societies majorly unravel. This War arc is and is not the end of the Hero society. By the end of this arc, all strings that form the fabric of the society will be loose and start pulling, but we will not see it fully come apart, just have it inevitably be pointing to that ending direction.
UA will "be fine". They will not attack UA, or stop them from having classes or anything. Why would them? Endevour will be exposed, exposing the loose morals of this society(or at least, perceived loose morals), and not one will trust their capacity to save them for even after pulling all resources together, planning, scheming, and surprising the enemy, thy still only got a thousands-of-civilian-dead
TIE. Their first public "tie" ever since Kamino, the true.... beggining of the end.
Now society will turn on itself, and the structures that holdit will start corroding. Maybe that's how UA stop being: Society asking the end of the madnessof training and using children as child-soldiers in a losing war for a unrealistic defeated society. Why Hero training when you wish to abolisih such a position?
Shigaraki will retreat and will not need to act anymore. The gradual bankrupcy of society is now on a steep slope that it cannot gain any footing anymore.
Eventually, sure... There might come a cataclismic moment to finally put an end to it all but... It is all already done, essentially.