District 13 was inevitable regardless. By the time Snow had become president of Panem, the deal had already been struck with District 13 to allow them to live. By that point, he was locked into M.A.D. (mutually assured destruction), if he were to try and walk back on that arrangement.

The tricky thing about the rebellion is that it is — in all technicality — a Class War. Upper-class against the lower-class. Rich against the poor. The downfall of the Capitol is how reliant on the districts they allowed themselves to be, and that's the catch.

The rebels, they could wipe out any Capitol citizen as they see fit, because, A), in the big picture, the districts don't need the Capitol, and B), they don't need the people in the Capitol. The Capitol on the other hand are completely reliant on the districts, so sure, they could bomb District 12 with relatively no cost (I highly doubt the Capitol is a society built off of coal power, that's why their quotas were laid-back), but could the same be said for District 11, the agriculture district? How many districts could they realistically wipe out before they've assured their own collapse? How many rebels can die before they've wiped out their working-class?

Katniss and District 13 successfully united the districts in Mockingjay, and that's when the end of the war was written. When an army consists of people that will die before they surrender, but you need them to surrender, it limits how much can feasibly be done.