You are probably rational enough to give up getting new kids if that means achieving longlevity/immortality. But most people aren't as rational as HN forum members.
Earth could easily carry 100 billion people, we could build cities on venus and mars, and if people could live for thousands of years, 100 year journey to another star on a slow ship would become quite acceptable.
Bedsides, In most places birth rate is already below replacement level, so longer lifespan is necessary to prevent population collapse.
No, it's not. Current estimates are that population will stabilize around 10bn people, not collapse: https://ourworldindata.org/future-population-growth
There would also be a necessity to allow (and even force) euthanasia for people in persistent vegetative state.
Source? Based on what standard of living/level of technology? There was even a time I thought Julian Simon had a point, but in the years since then everything I've witnessed has suggested quite the opposite. The more of us there are it's just as likely a tiny percentage will destroy the planet for the rest of us than it is a few geniuses will help us invent our way out of trouble.