Board Thread:Questions and Answers/@comment-33563373-20171104143048/@comment-33871937-20171206071009

I am about 50% serious when I say that there's a chance that One for All is actually the First Wielder, living on through his hosts as a twisted, parasitic entity. This lowered lifespan and limited amount of time to find a successor is due to the First Wielder siphoning his hosts' life forces, and subtly influencing their minds respectively. Inb4 the plot twist.

I am 100% serious, however, when I say that you're probably correct about the limited time left to live thing. All might is definitely going to kick the bucket, and live on in Izuku's mind as a spectre or vision from One for All to guide him in times of great need.