I love what they're doing with the MCU, but I do wonder about the long term viability, and how they plan on keeping it fresh and successful beyond a certain point. After about 3 movies for each major character, I think it'll be harder and harder to come up with really good films in the same setting and with the same cast. This is why studios tend to take a break and then reboot. I can see the MCU continuing to be very successful for a few years leading up to Avengers 3. But what then? After 3 Iron Man movies, 3 Thor movies, 3 Captain America movies, 3 Avengers movies, are they going to start making exclusively new launch movies? Is there much potential for Ant Man 2 or Guardians of the Galaxy 2, or Doctor Strange 2? Or are they seen as one offs? Lots of tough decisions for the long term business aspect of the MCU in the next few years I think.
I believe the whole plan is to keep building the Marvel brand to the point where they don't have to rely on the actors to bring in viewers but the shared universe itself. And they will get to the point where not every movie has to have direct ties to the Avengers movies, which we're already seeing with GOTG.
Basically, if they keep up with the quality of what we saw in Avengers and Phase 2 so far, people will go see these movies because they're Marvel movies, not just because it's Iron Man or Captain America or Thor. They can effectively give those characters a break (while still showing up for the big Avengers events) for a number of phases while they introduce us to Black Panther, Ms./Captain Marvel, Runaways, Dr. Strange, Ant-Man, Inhumans, Moon Knight, Blade, Punisher, (hopefully) Fantastic Four, New Avengers, Mighty Avengers, Nova, the list goes on. And they can still go back to the established characters for new movies that don't necessarily need to be trilogies - probably one of the reasons we saw them drop the numbered sequels for Cap 2 and Thor 2.
They seriously have decades worth of characters and stories to tell, they just need to keep up what they're doing and get to the point where they're not at the mercy of the actor or character's recognition in order to get people in the theatre.
They're in it for the long haul and I'd be very surprised if they ever do a hard reboot of the entire MCU. It'd break the whole reason they are doing the shared universe in the first place.