Quantum immortality sounds exactly like the mythical hell: living forever in perpetual agony, unable to die and in your own branch of existence separate from everyone else you ever knew.
What if we can hack quantum immortality to force continued good health, and the mutual survival of our loved ones in the same branch of the universe as us?
It seems like one would "simply" need a device which monitors your health with biosensors, and if anything goes out of range- it instantly kills you in a manner with extremely low probability of failure. All of your friends and family would wear a similar device, and they would be coupled such that if one person becomes "slightly unhealthy" you all die instantly, keeping you all alive and healthy together.
We nearly have the technology to build such a thing now. Would you install one in your own body? If not, why not?
Who wants to invest in my new biotech startup which promises to stop all disease and human suffering within the next decade? Just joking, there is a serious technical problem here that makes it considerably more difficult than it sounds: for such a device to work the probability of it's failure must be much much less than the probability of your continued healthy survival. You also never get to test the design before you use it.
But there is a question to dissect, even in this most basic of preferences. For example, I suspect that a large portion of your dislike for the prospect of being hit in the head with a baseball bat is the pain. Your objection, I assume, is not to the fact that wood (or aluminum, or whatever) will be touching your head, or to the proximity of an artificial object to your brain.
There is an aspect of the experience - being hit in the head with a baseball bat - which makes it undesirable where similar experiences are not. For example, I have far fewer objections to being hit in the head by an inflatable baseball bat, particularly if I am forewarned and the situation is appropriate.
Similarly, I would guess that there are particular parts of the experience of dying which persons like us would find undesirable (I clarify to distinguish rationalists from persons who may attach more superstition to death; we do not fear, for example, having our hearts weighed against a feather by the Egyptian deity Osiris) more than others.
In this case, I find it valuable to clarify: do we wish to avoid the experience of dying (which occurs with increasingly high probability in one's lifetime, even assuming quantum immortality), the limitations on the amount of fun we can have in a finite lifespan, or some combination of these and other properties of dying?
And yes, I did attach the adjective "undesirable" to death as a matter of linguistic convention. Thank you for pointing that out.