The Alien blood sends back memories. They explicitly state this. It's why the blood transfusion is a constant threat.
It's also implied that Emily Blunt had it way worse than Tom Cruise. There is a point where the two of them get stuck, and any choice made leads to her dying.
She tells Tom Cruise to get over it and keep moving, because she saw someone she was close to die in the same moment 300 times.
This is generally how writers cheat around the paradox. You don't send the whole person back, just the memories.
I imagine if Data hadn't whiz kid solved the problem in Cause and Effect, the universe would have eventually blown up. That being said, Time progressed outside of the collision, 4 months passed before Fraiser could ask for their insurance information.
whoa, just had a post movie moment. I thought the training portion of the film, where Emily Blunt kept killing Tom Cruise on the "Shooting range" was a waste of time and endangered them having the aliens come up with a new plan (They did) It is instead, Emily Blunt trying to make Tom Cruise self sufficient at the point she eventually gets stuck in an "always dies" situation. She needs him to keep going past that point.
(Up until that point, he had just been save scumming, memorizing enemy movements. He wasn't actually fighting the aliens properly.)
That being said, if the alien blood wasn't a completely magical McGuffin, yeah, he'd probably be ready for the old folks home. They never establish how long Tom Cruise is in the loop, but he memorizes some pretty inane poo.
Plus, Tom Cruise doesn't believe in psychiatry. Either PTSD or neural elasticity loss would be likely as the ole memory bank filled up. Then again, maybe it just overwrites the prior saves?