And you seem to be ignoring how flawed this JFK assassination scenario is. If JFK's assassination was the catalyst for the government sanctioned production and deployment of Sentinels against mutants it would probably be the slowest reaction to a perceived threat in history.
The Sentinels don't neccessarily need to have shown up in the 60's. I would imagine it would take some time to make sentient murder robots a reality.
They could easily just be the "Not So Distant Future's" method for solving the mutant problem since, due to the events of whatever the X-Men are supposed to stop from happening in the 60's, simply made things worse, and created an ongoing hate and fear of mutants and earlier and more extreme versions of the registration act/mutant persecution, VS a world that barely knew what mutants were in X-MEN, X2, etc.