My suspicion is that when they get to the point where they realise that the role is a burning dumpster and simply walk or switch role. Those that stick around and persevere are a pretty wild minority at this point IMO, I'm not going to deny that.
IMO it's a question that's progressively going to get more and more important as the steady influx of new players starts to dry up. Seemingly that's not an issue just yet, but that point comes for every MMO eventually and as it stands I don't think Yoshida will have an answer.