I don't think the plan/goal was to get to the highest tier of play. Not only do spellsand some abilities get extremely hard to implement in game (i.e. wish or similacrum), but one you reach that tier players should be dealing with cosmic level challenges, nothing that would be condensed to a specific city.
Also if they have balanced the experience correctly, it could definitely make sense that it takes that long to get to lvl 12. In my IRL campaign we just reached level 9 after about a year of weekly sessions averaging about 3hrs.
