Re: What would have happened if Avengers failed?
That's a damn good question, and, as your OP points out, shows just how much Avengers has changed the landscape for ALL superhero movies.
I won't go so far as to say that Marvel Studios would have just failed outright, but the focus would've gone even more Tony-centric than it already is. IM3 would've still made a buttload o' cash, but probably not the bill-plus it did IRL. RDJ would've probably decided that the "fad" had run its course, bowed out at the top of his game, and tried his hand at other material.
Singer shows no interest in returning to X-Men, so Vaughn gets his sequel, and it's just basically XMFC2, with no aspirations for any expansion of the cinematic universe beyond that.
On the plus side, in a lot of people's view, Fox would've probably been more open to selling off their Fantastic Four property to Marvel, and Marvel would've probably at least gambled on introducing them to the MCU.
Warners is stuck with permanent blinders on --- Batman on one blinker, and Superman on the other. World's Finest/B vs. S or whatever probably still gets greenlit, but Justice League, the Flash series, and even the Arrow series probably die in development hell.
THE COTTON AVENGERS
...They move like slick cotton on oil.