medical research might increase the quality of life, health-wise, but without concurrent improvements in society and the environment, it only means that we will die healthier in front of a firing squad.
Medicine cannot single-handedly solve all the problems. Of course it doesn't try to either. Yes, if everything else was wonderful, or at least tolerable for all, then perhaps the lengthening of lifespans and life quality would eventually cause food and space crisis. But since we are generally so good at killing each other off no matter how much we resemble a mangy dog, methinks it won't be an issue.
It would be more fun to ask how our societies would react to a sudden doubling of the average life span. If people knew they could live almost 200 years, would skydiving and other dangerous sports become less popular? Would society be able to concoct ways to change careers after turning 100, so that life wouldn't be quite so boring? Would the legal system invent 'divorces' for friendships because one or the other friend got bored after 150 years of hanging around together?
Your questions are legit, magic. Mine are too. Sadly, we are too good at killing each other, either directly or environmentally or economically, to need to ask any of those things.