Well, supposedly the way time travel works in the MU is that each incursion creates an alternate areality. If you travel to the past, you're not really travelling to 616's past but rather to 617's (for example) past, even though you return to the 616 when you return to the present. That's what confounded me so much about Young Avenger's fight with Kang and how the landscape changed, it wasn't supposed to. I guess it would definitely affect the people that did the travelling but I don't think it would change the course of history that much. It's a good thing you said time travel and didn't contain it to tech since Rachel, at least, WOULD have still been able to travel through time. I think that time would heal itself in a way that wouldn't be as radical as a What If?. That Black Beard trasure thing, would probably be replaced with something that they could have done in the present (sunken treasure in some unexplored place or something).