There are several issues that would need to be addressed imo if people could live forever.

Im not sure if I understood you correct, but imo talking about an "ideal number of next generation people" and sterilization is some sort of population control, and so the people need to be willing to follow these rules, and they also would have to follow, if we want a sustainable future. How ever having kids is an instinct, so that might be an issue.

The second thing that came to mind, is that I would vote for obligatory psychotherapy for everybody, because imagine some very powerful people full of hatred and power complexes (like we can see today)because of all kind of unresolved childhood fears and traumas, living to the age of 1500.

if we leave these problems aside, I think it would be a very interesting experiment, to see what kind of wisdom a human that old can develop.