It's not about the other realities still existing. It's about getting to the good reality and leaving the bad reality. Relativity at its most literal. Where is the observer.
In many-worlds, the observe, you, end up in *both* realities. That "you" avoid the bad reality is an illusion, an artifact that we write stories from a single point of view, and we choose to follow the one who ends up in that one reality. But, since the other timeline still exists, you are in the bad one as well. In many-worlds, "You" are not a single stream, but a tree with many, many branches.
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