It's not about Kennedy, specifically. It is about the butterfly effect. Change the past and literally ANYTHING can happen…which is why the past was pushing back that entire time. It did disturb me that we never found out what happened to the girl who crashed into the phone booth. Did Jake call 911 or just take off…

But he could have still been alive in the alternate future. Since the diner wasn't even there, one can assume that in that version of 2016 he never would have found the wormhole, therefore he would not have gotten cancer. So having him show up instead of or alongside Harry would have been interesting.

Exception: "election." Movie was twenty times better than the book. Otherwise, u r correct.