> Getting older and eventually dying is imho, what defines our humanity.
Maybe, but is that a good thing?
To simply "expire out of existence" is a big issue for "humanity". It's the main reason why there's no proper answer to "what's the meaning of life" - whatever you do during your 50/70/120/.. years of life you'll eventually end up just as dead as the guy right next to you. Congratulations!
Being immortal would allow us to get rid of crutches like religion and allow us to implement proper moral systems, since there would be tangible rewards to being "good" simply for "goodness' sake", since being good and being selfish would be identical - if you'll (eventually but reiably) suffer overpoputlation, pollution and even unemployment just as much as the next guy, you'll (eventually but, again, reiably) think twice about elbow tactics.
> Oh yeah, I really doubt this - I know a lot of older people and most of them would not want this trade in.
20? Maybe. but How about being in the body of a 30- or 40-year old again? I can't imagine any 70-year olds not wanting to have a more "hassle-free" body, all other things being equal.
> makes live unworthy
Talking about live being "unworthy" is a slippery slope, but there nevertheless is an argument to be made about quality vs quantity of life .
I don't have the reference (and the exact numbers) at hand, but in "Happiness Hypothesis" (the Book) the author mentions a survey where people got to express their preference between "living x years of life and then dying to some tragic accident" and "x+y years of life, but y years spent in a wheelchair, because of some tragic accident". The results were rather mixed with the shorter life being more popular choice.
And, by the end of the day, a chance at higher-quality life in exchange for possibly less total life is also why things like armed robberies exist (even if not every robber things of it like that beforehand). I'd also argue the same is true of people opting for high-risk jobs, but there's also a sense-of-duty aspect to them, so it's not quite as clear-cut there.