Time Machine Chaos

What if everyone had a time machine on their wrist and could go backwards (not forwards) in time to make whatever changes they needed to in order to make their lives better, or perhaps make other people’s lives worse? The could then come back to the present.

Imagine making a change and then seeing that change unravel as someone else goes back to change your change to favour them!

With an infinite number of possibilities, what sort of temporal chaos would ensue? Would the “present” simply become untenable? Would the future be beyond incomprehension as to cause insufferable anxiety in the people of “now”? Or would they understand that not only is the future not written, but the present is so malleable that it makes no difference what we think the future is going to be, someone will inevitably change it anyway. So stop worrying about it!

If everyone had a time machine, would Hitler be in a constant flux of existing and not existing? Would Marty ever actually make it back to 1985?