This isn't even true in anything but a irrelevantly literal way, first of all. There are incurable diseases which can cause massive suffering. It would be irrational to live with one when consensus is that a cure won't be remotely on the horizon for one's lifetime. Sure, something could come up unexpectedly but you could also win the lotto too and it's irrational to play that. Second of all, your argument invalidates all decision making and planning, so is really a reductio ad absurdum for itself.