What does not wanting to be mortal have to with utilitarianism?  Note that many mainstream religion members obviously also want to be immortal, they state they believe that a higher power has already made them immortal and their actions while alive determine their future situation after they die.

I'm pretty sure most people here are utilitarians and also want to be immortal, I'm not sure why there would be a contradiction between those two things. If the claim is that most here "just" want to be immortal no matter the cost and don't really care about morality otherwise, then I disagree. (plus even that would technically be a utilitarian position, just a very egoist one)

Ok, in my prior replies I confused "utilitarianism" with "consequentialism".  

In this case, I think you are trying to say that individuals being immortal is somehow contradictory with "the greatest good for the greatest many"(utilitarian).  This is a typical deathist belief (a deathist is someone who is pro-'natural death').  The implicit subtext is that "to give someone else a chance to live...", since all ecologies are finite, and people not dying closes slots other living humans could exist in. 

 Or if the medical treatments were extremely expensive in real cost, say if it took the labor of 4 doctors solely dedicated to one patient to keep them alive indefinitely.  But that's not a likely scenario, a medical treatment that reversed all the deleterious changes from 'aging' would likely make each treated patient cheaper to treat than untreated patients their same chronological age.

There are issues with retirement systems ('should they pay based on chronological age or biological age') and breeding rights ('should someone reversed to a fertile biological age be permitted to breed as often as they want') but these are kind of minor issues.

However, the many is composed of all individuals.  Almost all individuals seek to be functional and alive most of the time.  So it satisfies the personal good/preferences for most members of the set of "the many" for them to continue being alive and not crippled or deteriorated from aging.  (most of us don't just want to be immortal, we want to be eternally young or post human, and our personal survival changes are maximized in a world where a powerful entity like a government grants the treatments to everyone).  

The looming fear of death also deteriorates the life quality of all members of the set of "the many".  So if you believe it is morally right for all 8 billion humans alive to "pass away" and replace them with 8 billion new humans ("the churn"), those humans will also desperately fear their own deaths, hate aging, and so on.  You haven't really improved anything.

Finally, the strongest argument is that if you are a deathist, and you get enough of your friends to vote, and you cause aging treatments/ASI to be delayed sufficiently that every human alive right now dies, you only killed yourself.  (and murdered a few billion others).  There will be a future generation that tires of "the churn" and makes themselves immortal.  (notice that an AGI takeover is an outcome in that class - AGIs are immortal inherently)