Salvation is both individual and corporal. This means that it is not enough that you go to heaven, even for you; enough people must be saved in toto.
This is because the war between humans and angels on the one hand and demons on the other can only be won decisively if a sufficient army is assembled. It may even be that saints are simply reincarnated again and again until either a final victory or final defeat is reached in the overall war.
We are all our brothers’ keepers.
How will that final battle look? It will of course not be physical, since demons are incorporeal. And it will not be primarily intellectual. The intellectual victory belongs to the individual who sees the truth and is not in this life deceived by the demons, not to the whole army.
Rather, it will be spiritual, involving our wills and the charity within. In short, the love with which the human race and good angels will be bound will literally crush the demons and drive them into hell.
But as of now, there are neither enough people nor enough charity in their hearts to do that. So the world must endure until the divine project is complete.
Who will win? We do not know that by natural reason, and in fact according to reason there is plenty of room for skepticism and even despair. But we do know that we will win, eventually, by faith, since otherwise Christ would not have risen for us.