We don't know which is the "last" of anything until quite a bit of time passes afterward. The last human born will be someone we will never know about, because that means the human race will die out. Kurzgesagt doesn't have all the answers, but presents some important ideas in a thought experiment. There are a lot of variables in even speculating about a species dying out, especially the one species doing the speculating. We are currently doing our best to destroy our earth, but we are also exploring the idea of living on other worlds. We may slowly (or quickly) evolve into something that is a bit different, but more likely to survive.
While it may be depressing to think about the end of humanity, the best case scenarios are unimaginably far into the future. Kurzgesagt is actually quite optimistic about our future, if we can make it through the next couple hundred years. The subject is only ten and a half minutes long, the rest is promotional.