Even without aging, humans aren't immortal. There's a probability of dying from something other than medical issues. Eventually that adds up.
I think the people willing to give up travel at greater-than-walking-speed, sports, hiking, etc in order to live longer is small enough that they won't be a burden. I'm not sure I would really call that "living" myself, but it's their choice.
Totally not reliable, but I once heard an estimate that if aging was cured, the average lifespan would be about 600 years due to various forms of non age related death-- accidents etc.
It’s a group survival thing. It’s helpful to have some reckless members because they discover solutions that others would never explore. In particular, young males have been prewired by evolution to generally be the most reckless members of a group because they are the most expendable. The number of young females determines a group’s reproductive rate and older members tend to have critical skills, knowledge, and wisdom. I speak as a former young male who explored more than his share of possible solutions for the group.
Presumably though even if biological aging were somehow paused at the level of young adulthood, I would think cognitive development would still proceed, so young males would still outgrow that reckless period.