Okay, so here's a weird scenario for you. Time travel is discovered, but it's not what everybody has thought for years. It's basically interdimensional travel. Which means that if you go to the past, change something, then return from whence you came, your timeline isn't changed in the slightest. What you've done instead is create a new timeline. You can literally go back and save Kennedy, kill Hitler, or whatever, and your original timeline is perfectly safe. Nothing has changed. You could kill one of your great grandparents and have sex with the other, and no consequences in your original timeline. Alternately, you can change things and follow the new timeline back to the moment when you left your original timeline to see what's changed, then go home to the timeline where it never happened.