They did it for two reasons. 1) People were asking for high level/high power content. 2) They have already filled in all the basics. All the bases are covered with the current classes. The same goes for spells. After you have put out an "Ultimate Equipment", what are you going to call a second book of equipment. Weird races are done. So, the tropes left were epic, in which they twisted a bit and made mythic, psionics, and robots. They are quickly running out of topics to create rulebooks for. Hopefully, they realize and accelerate their AP production to increase adventure content. They are almost done writing rules and need to focus more on writing stories. The focus on Pathfinder Online launch will provide a bit of distraction for a while next year, but the horizon is fairly clear.
Yeah, attaching access to mythic to DM credits will slow the proliferation of mythic characters quite a bit, as well as their growth. We currently have five GMs that could spend enough DMC points to get to 3rd tier, and only PM could get up to 4th level, if we used the sliding scale I suggested in the judges forum. Those GMs have been doing the hard work continuously for over a year now. But, anyone who is interested in Mythic, after being a GM for 90 days, will have earned enough DMCs to be able to join the mythic club. So, everybody can have access if they put in the extra time to be a GM.