[theamk]

I don't why biological limitations restrict humanity's long-term future.

We are already ignoring them - right now it's freezing outside, and no human could not survive in such weather using biology alone. I am also living in a city which is way too dense to sustain natural human society.

There is always a risk of entire species dying out, for example via global war, or everyone suddenly deciding they don't want technology and then freezing to death in the winter; but there are no universal long-term limitations. At some moment there will be independent colonies at other worlds, and then humanity will be eternal.