Mythos cap is a way to force people to continue paying month to month. If I unsub and wait until more content is released with 2.1 or 2.2 to resub and there is still a 300 mythos cap, then I missed out on a great deal of mythos. If there is no cap, I can unsub and resub freely and won't have a limit to how fast I can catch up.

Basically the only reason I don't unsub is because I don't want to be out 300 mythos/week until I resub when they add more content because IMO the endgame is incomplete.