Even without proton decay, things don't last forever. You can sort of justify that just by thinking about entropy. So i mean, it's a little slightly casual aprox a statement when we say everything falls into black holes but it's not doing violence to the ultimate conclusion you draw from this. I think that ultimately, if you have one single reason why it was successful in terms of people believing that this was a good thing to do, it's because it worked. This kind of reasoning paid off, paid dividends in terms of a better understanding of the universe.