Many people quit at max level and walk away. I have seen much discussion about a lack of end game content, unwillingness to start the treadmill over, lack of direction without the treadmill, and a few others, but there is one explanation I have heard rarely if ever: people get sick of the treadmill well before 50, but tough it out to “finish the game” before they quit.
Before housecleaning, I would look at our CoH SG and see level 47-49 characters who just walked away one day. “How can you get that close,” I wondered, “and just let it go?” Then I remembered, wait, I did exactly the same thing in Dark Age of Camelot. I quit a few levels shy of the level cap, and I would not have gotten that far if my class were not in demand AFK/borrowed dual box (Earth Theurgist with bladeturn chant). It is easy to get sick of the level grind, and many people have quit in their minds but just want to “beat it,” to say that they did (to themselves, mostly).
For a variant perspective, consider studies in the timing of death. People really do hold on for that birthday, new year, grandson’s graduation, or change in inheritance laws (check Ig Nobel prizes for that last one; it actually is solid research). Yea, the soul-crushing agony of the level grind unto death.