Correct me if I'm wrong, but this would only "prove" the existence of (that variety of) multiverse in a very small subset of universes.
So, let's say I try to poison myself with a pill from a bottle containing 99 cyanide pills and 1 sugar pill. There is a 99% chance I'll die, and a 1% chance I'll live. So in 1% of all universes, I live. I repeat the expriement multiple times, until only 1 in 1 million universes has a surviving me in. That means that in 0.0001% of universes, a very smug version of me is winning a Nobel prize for proving the existence of the multiverse. In 99.9999% of universes, I am dead and nothing has been proven except that I really shouldn't be allowed access to the lab's supply of cyanide pills.