If the show was reality, then yeah, it turns out they didn't really need to escape the island that second after all. But since it's a show, and they needed to tie things up for us at the end, then:
1. Jack needed to know they got off the island before he died, and
2. WE needed to know they got off the island so we'd know they lived out their lives before they met again at the church.
We weren't going to have time to find out whether Hurley COULD change the rules and, if he could, how and when they would get to leave, etc.