There's no denying that careful planning and an extraordinary amount of luck went into the creation of the MCU. Several other factors went into the success of "Phase One," as Marvel Studios president Kevin Feige calls Iron Man through The Avengers (we're currently in the midst of "Phase Two," which concludes with The Avengers: Age of Ultron in two years). First of all, the superhero film was in the midst of peak popularity, with Christopher Nolan's Batman films earning critical raves and boffo box office and Sam Raimi's Spider-Man films smashing records. Even though films such as Fantastic Four, Daredevil, and Hulk had not exactly performed well, it seemed like audiences might be willing to turn out for films about unfamiliar superheroes. Then there's the casting: the role of billionaire playboy Tony Stark and Robert Downey Jr., an actor who was trying to recover his career after flaming out as a hard-partying star in the '90s, was a marriage made in movie heaven. Iron Man became a huge success mostly on Downey Jr.'s charisma, and Marvel was able to plant the seeds for the future. Attracting more major stars - Samuel L. Jackson as Nick Fury, Scarlett Johansson as Black Widow, Chris Evans as Captain America - gave the films even more drawing power. Lastly, Marvel was willing to create a continuity within its films that was separate from those of the comics. This was an absolutely necessary move, since the movie rights to other Marvel properties such as the X-Men, Spider-Man, and the Fantastic Four belong to studios other than Paramount, which has partnered with Marvel Studios for the Avenger films.
So that's where we've come from; now where do we go? Luckily for Marvel, they have a lot of advantages that they can put to use to keep the MCU afloat. The first is that superhero movies are still doing phenomenally at the box office. This year alone, Iron Man 3 had one of the biggest opening weekends of all time, and is currently the year's highest-grossing film (topping out at a little over $409 million), while Man of Steel and The Wolverine both topped out at over $100 million. There's no real sign of audience fatigue yet, and each new announcement for the next big superhero film invokes enormous reactions online (not that that necessarily translates into box office dollars, but it doesn't hurt publicity).
Secondly, as fans of comics know, superhero mantles are often embodied by more than one person over time. Peter Parker isn't the only Spider-Man, for example. This is important in that the actors who are currently embodying the characters won't always be involved; Downey Jr.'s current contract stipulates that he only has to appear in two more Avengers films, and [SPOILER ALERT] Iron Man 3 insinuated that Tony Stark may be stepping down from being Iron Man. If Marvel wants to keep going, it's going to need to be able to replace the actors playing the superheroes effectively, or else risk running through a roster of increasingly-obscure replacement superheroes.
On a related note, this can be done by bringing the comics' concept of rebooting the universe to the MCU. In Marvel's comics, most story lines exist within the "multiverse," with alternate realities where the same characters exist in different forms. This allows for more possibilities for the existing slate of superheroes. Say, for instance, by the time The Avengers 3 rolls out, the MCU is running out of time with the current cast and characters. If the film were to present a narrative that introduces a rift to an alternate reality, al a DC Comic's famous "Crisis on Infinite Earths" arc, then the MCU could reboot itself with all-new actors taking on the familiar roles. This way, the films can overcome the reality of cast shakeups and possibly keep audiences engaged.
Unique to the MCU, though, is that Marvel Studios is willing to take creative risks within their blockbuster properties. When Jon Favreau signed on to direct Iron Man, he was best known as the actor-director who starred in Swingers with Vince Vaughn and directed Elf, starring Will Ferrell. His resume didn't exactly scream "$140 million superhero film." The same can be said of Kenneth Branagh, who before Thor was best known for starring in and directing Shakespeare adaptations, Joe Johnston (Captain America: The First Avenger), who was responsible for Jurassic Park III, and Joss Whedon (The Avengers), the cult-TV phenom (Buffy the Vampire Slayer, Firefly) who had previously only directed one feature film (Serenity). What's more, each of these directors brought with them a sensibility that set them apart from the traditional superhero film. Branagh elevated the Shakespearean family politics of Asgard, casting Loki and Thor as tragic figures in a struggle between two worlds. Johnston brought a whiz-bang sense of fun to Captain America, presenting it as a rah-rah WWII adventure film. Whedon made The Avengers into the quippy, exciting ensemble piece that it needed to be in order to live up to the hype. And there are experiments in genre, too: Shane Black fashioned Iron Man 3 into a buddy-cop film with noir-ish undertones, while Thor: The Dark World (directed by Game of Thrones vet Alan Taylor) looks more like a fantasy and Captain America: The Winter Soldier (due next year and directed by Anthony Russo & Joe Russo) has been described as being inspired by '70s conspiracy films. That's not even counting indie horror director James Gunn taking on Guardians of the Galaxy and Edgar Wright - he of Shaun of the Dead and Hot Fuzz fame - being attached to Ant Man. Utilizing directors with fresh takes on the genre will help keeping MCU films from becoming hackneyed and uninteresting.
Guardians of the Galaxy concept art
Simply put, the MCU won't last forever - at least not in its current state. For now, though, it continues to thrive, with the building blocks in place to create a long-term future for the films. Whether that future materializes remains the question.