We have as many reasons to believe that the legacy of the Roman Empire, through the Catholic Church, still exists today and is a billion people strong as we do why it fell at any one particular day for any one particular reason.

i can't parse your comment, but it seems to presuppose that the roman empire fell on one particular day; there seem to be a lot of candidates, though. which one do you mean

There are several days that could be candidates for the day Rome fell.

There are several reasons we could argue that the Roman Catholic Church is the continuation of Rome and so Rome never truly fell.

That is what I was trying to say in the previous comment.

it probably makes more sense to argue that the byzantine empire is the continuation of rome, but in a broader sense, all of human civilization is the continuation of the indus valley civilization

The point being there isn’t a single cause to the fall of Rome and there are even a few arguments that Rome continued in various forms, some of which still exist today.