Those are obviously problems, but there are solutions. Among them, "everyone dies" is just about the shittiest. Yes, I would happily volunteer to be sterilized in exchange for living forever. In a heartbeat.
Also, we are nowhere near being immortal and in many developed countries people already have stopped reproducing (Germany, Japan, Spain, etc)
A world without kids and young people is a really, really sad state of affairs. I don't know and I don't care about what others would do, but I think I will just choose to end my own life rather than continue to live in such a dystopian future.
1. There wouldn't be zero kids and young people. Just not very many. The death rate will always be non-zero, and our ability to support new people will grow over time.
2. I get that no more children is a bummer, but I'm not really understanding how it is so awful that it could be described as "dystopian", much less something to kill yourself over. The world is vast and interesting, and children are just one neat facet of it.
In any case, I can see how it might work to give people a choice. If you'd rather die sooner and also have children, you could opt out of both the sterilization and the immortality.
Then, ok, fair enough. The question is, how do you make that fully general so that our ecosystem doesn't collapse and kill the lot of us?
And then, how do you make the economics work out? Immortal transhumans need to eat, too, but once they get some kind of capital base going that expands faster than inflation, as long as they've got even the barest livable income, they'll eventually live long enough for their wealth to grow into "OWN EVERYTHING EVERYWHERE" levels.
And we thought today's asset bubbles were large!
Meanwhile, the young of the future will be even more screwed than we young are today, for precisely the same reasons but more so.
It's similar to an argument people sometimes give me when I tell them I'm vegan, which goes along the lines of "If everyone suddenly became vegan, imagine what that would do to the world economy! How would we feed all of these people when our food producing infrastructure is animal based?"
And the reason I don't worry about that is that I know that scenario isn't going to happen. When people stop using animal products, which I think will happen - not for ethical reasons, but for economic reasons as cheaper, more authentic substitutes arise, and the sustainability of animal farming dwindles - it will not be an overnight process, and the world will have plenty of time to adjust.
It's basically the same with extreme longevity. We don't have to solve these problems with the tools we have today, because we don't even know when they will be problems that need solving. And we have no idea what tools will be available to us once we actually do have to solve those problems.
But if you flip the scenario around, and imagine that everyone already lives forever, and these problems start to show up, do you really think that anyone would even think to suggest "Let's have everyone die after around 70 years or so"? That idea would be grimly hilarious in a very Modest Proposal sort of way.