I feel that the problem with the FF is that it is all about family. This does not sit well in today's society with many relationships fractured and people in multiple marriages etc. I am not saying that this in itself is wrong but I feel that the FF represent a stable unit, which is not an idea reflected by society, unlike the Avengers' and X-Men's fluid roster.
So in short it is the subject not the characters.
There are loads of movies that celebrate family, friendship and so on. Most Disney movies do it. Many movies in general do it to varying degrees and in different ways.
I am not sure that your hypothesis has much merit to it. Other than a negative view of society. Movies about family are still pretty major draws if they are done well.