Makes no sense

So I assumed this was going to be horrible, but there are some really random moments/jokes that made me laugh. The flash with the daughters telling him to make Kanye money, Farley's parts, the yanking Doc, the deserted island and realization, Walken's laugh, anyway, it had its moments. The issue that I don't get is why isn't he 80 years old after living through the extra lifetime, and why would his time travel destroy his past self from meeting Brandy...by that logic he would have never existed to begin with...