The only reason population numbers aren't collapsing is immigration, but that comes with its own issues. Extending live expectency and productive lifetime isn't that crazy (if we can get a handle on our rising living standards and resource consumption).
People who are physically young are at a much lower risk of cancer, cardiovascular disease, Type II diabetes/Alzheimer's, etc. Stopping/reversing the aging process kind of fixes the root cause of a lot of those diseases.
We're probably not far off from that reality, either. Then we move onto more controversial questions, e.g. Do physically healthy people have the right to end their own lives? Not everyone want to live to be 300 years old, especially those who have lost most of their family and friends over those years.
I've experienced extreme loss, but continue to expreience new connections, knowledge, etc. There's simply no small limit to what a human can experience, and hundreds of years is paltry compared to when it would take to even be on the verge of boredom/emptiness, if such a verge could actually ever be reached.
If my kids, or their kids, or their kids, etc. ever wanted me to die in order to "bestow" them with something...well, too bad. I wish my grandparents, and their parents, etc. were alive today.
Eventually all the atoms we know and love will decay and "disappear."
If we get to a place where people can live healthy lives until they are 300 years old, why do you think someone who finished college 40 years ago will be more valuable for a company than a fresh graduate? Because nowadays we have exactly this setup minus the 300 year lifespan, and often times the company is happy to let go of the senior guy.
You don't know that. You think that is the case, but (I) you don't know what death is like, so you cannot realistically claim it is worse than anything (II) until you qualify 'fucked up' then this is a pointless claim, since your version of 'fucked up' may not be that bad compared to what is in the realm of possibility.
It's incredibly more likely that continued existence will be no worse than the world today, which is quite nice. Even if it becomes torturous existence beats non-existence.