I think people are naturally beginning to want an E Ticket attraction. Every decade+, Disney goes on a building spree that results in multiple new E Ticket Attractions. Back in the mid 2000s, we got one of the most prolific periods of consistent new E Ticket building. Expedition Everest, Soarin, and Mission Space all opened in a few short years. *
This particular boom phase helped support a massive increase in Ticket Prices over the last decade or so.
However, it's been nearly ten years since Everest Opened. Guests have had to bear a pretty strong increase in the price. For the last few years, WDW has been coasting. That's fine, but it's in sharp contrast to their competitors who have been moving at lightening across their Resort.
Once again, I don't begrudge them for wanting to get ROI. I just don't think it's sustainable for much longer. Disney can't keep going at this trajectory. That's why, starting in 2017, we're going to be getting a new burst of E Tickets.
While at first I was rather dismissive of the Slinky Coaster, it looks like it should be near E Ticket Caliber (admittedly optimistic) upon further inspection of the concept art. It really does look awesome.
So Avatar/Slinky for a 2017 debut is great. That will start the ball rolling in the right direction. SW will hopefully debut with two new E Tickets in 2020ish.
Now there is a question I'm starting to ask...
Is it enough to sustain WDW on another course of ticket price increases into the 2030s? That I'm less confident of.
As for the roller coaster v. Other types equaling E Ticket, I think you'll find lots of people who are sympathetic to your point. Rides like Pirate of the Carribean or Haunted Mansion could be considered Es. I don't think any Disney fan thinks that Roller Coasters exclusively represent E Tickets.
I would say that they're the predominate E Ticket being built today though. When a big budget project is approved, they will typically spend that on a roller coaster because it has wide ranging draw.
Also Disney, sadly, has been less willing to spend big money on dark rides. Which is a shame. That's beginning to change to an extent though.
Disneyland has just reinvested into their lineup of Dark Rides, and Disney built Little Mermaid. While it has been criticized for that attraction, it's still solid. I think it has a place in the park. It was overhyped though.
I also think that a lot of the complaints come from people who have seen the Tony Baxter version dreamed up for Paris...
*some like to dispute whether those are really "E Tickets." While I do agree that criticisms can be levied there way, especially in their implementation, no one can deny that they were significant draws that have become some of the most important attractions at their hosts.