Suppose at some point all diseases and sicknesses were curable. (Assume that death due to starvation or thirst or being fed into industrial sized blenders were not curable.) Assume further that people do age and grow enfeebled, but just don’t die. What’s the practical impact?
ISTM that it would be a disaster. If people stopped reproducing almost entirely, then you could keep the population stable, but there would be fewer and fewer able-bodied people capable of keeping things running. It would pretty soon reach the point where the entire system breaks down and everyone starves to death (not to mention any number of other catastrophes which would arise).
But even if people keep reproducing - and it’s hard to imagine that reproduction would cease entirely - then that only lessens the problem, and doesn’t eliminate it, since the percentage of feeble people would greatly increase. And meanwhile you’d also have another problem, in that tremendous population growth would severely strain the planet’s resources.
Bottom line is that if death from natural causes was solved, then the only way to avoid mass suffering and death from unnatural causes (calling starvation and the like “unnatural causes” for this purpose) would be if both 1) natural senescence was also solved, and 2) reproduction was drastically curtailed.
Medical science is obviously nowhere near solving natural death at this point, but the above might be interesting as a thought experiment. But more importantly, where I think it’s significant is that as more diseases and illnesses are ameliorated to one extent or another, the above issues arise, though to a much smaller degree. Essentially, as long as there’s a disconnect between science’s ability to keep people alive and science’s ability to overcome natural senescence (and declines in natural reproduction) then there’s an increasing strain on the workforce and natural resources available.