I really hope someone will do some serious philosophy about Quantum Immortality soon. The only thing I've seen is an article (a very pessimistic one) about it by David Lewis from about 15 years ago. It just feels way too important to be left to random lesswrongers to speculate on.

Is this a correct summary of your idea?

"If MWI is true, then all the possibilities in which the universal wave function can split are realized, and there will be branches where I live for a very, very long time.
Out of those branches where I am 'quantum immortal', those where the immortality is due to normal occurences will be much more probable than those where the law of (classical) physics are explicitly violated.
For example, in the branches where I do not die hit by a bowling ball in the head, those where I simply decide not to go bowling are more probable than those where the ball simply stops mid-air or tunnels through my body."

I don't think advocates of QI generally mean by it what I think you're taking it to mean.

My naive linear model is that ~$400 billion research funding currently spent per year buys about 1 year increased lifespan per decade, so it would take about $4 trillion per year spent on research to stop aging, or a one-time investment of $80 trillion. For 99% confidence I'll add a safety factor of 4, yielding a one-time payment of $320 trillion, or $16 trillion per year. In other words, this back-of-the-envelope guess suggests the entire economic output of the United States would be just sufficient to discover and maintain an aging cure.

Can I recruit followers? Starting a cult is a useful exercise for ambitious rationalists.

my consciousness observes exactly one world at once

On macroscopic scale, yes. Trying to observe a particle in the double-slit experiment could change your mind (you might realize you are observing an interaction of multiple worlds, differing only in the trajectory of the observed particle). This is probably irrelevant for everyday life, but using the word "quantum" reminds people that it's at least technically not true.

It could possibly become more relevant in a very far future, approaching the heat death of the universe, if we take the immortality literally.

my argument is that Quantum Immortality works backward in time, if that makes sense.

Seems to me like an example of selection bias. Some of your future you's will die, and the rest of them will happily exclaim: "I knew I was immortal!" The question is whether this is a "correct" way to describe reality (and what specifically "correct" means in this context).

If, hypothetically, I tried to catch a terminal-velocity bowling ball with my face, your theory says I would experience the bowling ball doing nonfatal damage and then stopping just before killing me, and my theory says I would experience changing my mind and getting out of the way of the bowling ball.

So from the perspective of a you that I can talk to after the near miss with the bowling ball, your description makes sense. But it also makes sense to me. We are both in the universe where you changed your mind before the bowling ball hit you and you got out of the way.

But from the perspective of me in the world where you got hit by the bowling ball and died in pain, your consciousness did whatever consciousnesses do when people die. Presumably it felt the fear when it noticed he inevitability, felt the impact and then the pain, and then stopped working as the neurons in the brain stopped working, some from immediate injury, others more slowly form loss of viable environment.

The worlds in which people die exist. I am in a world where billions, of people have died. A small number I have seen die with my own eyes, a larger number I have seen soon after they died, a much larger number I know of by reliable report.

This immortality you speak of: if there are identical twins and a the age of 5 they are crossing the street and one is hit by a bus,has not some individual died? If you live in a world with MWI, and at the age of 5 for one conscious version of you the universe splits, and in one of those branches EVERY new universe generated ends in your death at a finite age at least 20 years later, while in the other branch there are some branches where you go on forever, than have there not been at least one conscious version of you which will last 20 or more years, but not infinitely, that will die?

This idea that your consciousness jumps from the dying world to somehow mystically join with the version of you in a different world is anti-intuitive at best, and non-scientific or religious at worst. Nothing else jumps between worlds once they have split, why would consciousness? There is already a consciousness in the world you want to jump to with different experiences than yours as you face your last seconds of life, how is there room for your consciousness to pop on over to the other universe to escape death?

Your theory strikes me as the opposite of timeless. Your theory seems to come down to, if I ask my 10,000 year old self about the worlds, I am always going to get an answer in which I lived at least 10,000 years. But if you ask your 20 year old self about the world, then almost all the answers you get are going to be about worlds in which you live less than 100 years, I say that based on the observation that the people other tyan you that you see, way over 99% of them are dying before age 100.

A QI belief in infinite life seems indistinguishable from any other regligious belief in infinite life, at least in regards to conformity with evidence, logical plausibility, and some amount of wishful thinking.

it's more probable that I'll live forever from the World I experience now

This also explains why you find yourself in a world that has already perfected immortality technology.

... Oh, wait.

[EDITED to add: I see that you sort-of addressed this "... would require a more complicated Universe". But I don't understand that at all. How would it require a more complicated universe?]

Well, the nice form you described here doesn't work. The kind of lousy usual form does, with the usual caveats.