A thread on the Marvel Heroes forums over the weekend discusses how the game's priority has been to add new stuff to sell - playable heroes, costumes, and now new NPC "team-up" mercenary-like heroes - rather than new zones. That part is obvious enough - you are what you sell, and Marvel Heroes is not in the business of selling new zones, nor could they make reasonable revenue doing so given how long content lasts in an ARPG. The part of the discussion that I find more interesting is what they have done instead.
Since launch, the game has added a wide variety of new item slots or game systems that increase character power and are NOT tied to specific new zones. Whether it's synergies for leveling additional characters through existing zones, runes and enchantments as world drops for farming existing zones, the new team-up characters to accompany you through the existing zones, almost nothing on offer requires a specific type of content (some of the rarest artifacts and rings are the exception).
I'm not sure what to call this steady inflation to character power. It's arguably not vertical progression since characters levels are staying the same and you aren't replacing one system with the next (i.e. you don't stop using synergies because you've gotten runes and those are better). It's also arguably not horizontal progression in that the relative level of character power compared to the content - mobs have stayed at roughly the same stats and thus gotten progressively easier - is skyrocketing in a way that I find concerning. Will players in 2017 zone into the newbie area, click their basic attack, and one-shot Dr. Doom from eight story chapters away?
There are many differences between an online ARPG like Marvel Heroes and a traditional MMO, but in some ways I wonder if this is the logical evolution of the theme park non-subscription model. Don't be in the unsustainable business of trying to sell content - see version 1.0 of SWTOR. Give away the content (roughly the equivalent of a single player console game) and make your money selling stuff to do in that environment. Focus on replayability, accept that your most dedicated players will burn out and/or trivialize the game, but hopefully return after a break.
Not saying this is better or worse, just different.