Avengers is more of an unknown quantity. They should not be trying to make a movie for comic fans, they should be making a movie for the wider audience that is also enjoyable for comic fans. I think they might have bitten off more than they can chew...
I know this was posted a month ago, but...
Future X-men movies will NEVER become as big as The Avengers have been.
Yeah they've handled the franchise really well so far. Running the solo movies in parallel is also a great way to keep the ball bouncing without 3-4 year gaps too.