Yes, but do these alternate histories exist as physical realities? If they do I would agree with you that they are full of paradoxes. The simplest example I can think of is an alternate physically real history with a law for existence that postulates the impossibility of alternate histories. That would make this future impossible because we postulate that alternate histories exist...unless we are the physical evolution of that history (the one that prohibits alternate histories)
If these alternate histories are merely 'thought experiments' meaning they don't physically exist then what is the point of the underlying philosophy of Quantum Mechanics? You don't need Q.M. to explain that an experiment can have many outcomes. In my opinion, what Q.M. contributes is the idea that an experiment can have ANY outcome and all those outcomes EXIST until you collapse the wave function by actually performing the experiment. In my opinion this is an absurd idea filled with paradoxes.
One more thing. Popularizers of Q.M. always mention the POSITIVE aspects of multiple realities. You are rich, beautiful, healthy, smart, safe, etc. They never mention the negative. You're missing a leg, an eye, you're constantly tortured, murdered, etc.
If you made a list of the positive and negative aspects of multiple realities, the NEGATIVE would far outweigh the positive simply because there are more ways to cause chaos than there are ways to cause order.
Personally I wouldn't like to be a rock. I like the fact that billions of years of evolution have 'steered' me to complexification and greater order as a conscious being able to think my own thoughts.
These multiple realities Q.M. is allowing... most of them would be quark soup...if you follow the logical consequences of the reasoning of popularizers beyond their first few sentences.