That is true but Marvel had not gotten to the point of second or third stories in the case of many of these films, I think it's safe to say they would have fallen into the same rut without Disney's help, all other studios do.
The saving grace for the Marvel franchise in general is the sheer number of solo characters they have to choose from for standalone projects, like the upcoming Doctor Strange, Gambit, Sinister Six, Deadpool, Suicide Squad, Black Panther, Captain Marvel, Inhumans, you get the idea.
They do far better when they are not trying to maintain a current character set in a new story, like the Avengers but instead can just focus on telling the untold story of a new character which is the piece that is missing from sequels unless new characters are introduced.
New characters in sequels is something they seem to shy away from in favor of what I'm sure they see as showcasing existing talent to capitalize on that angle. If they do intro new characters they tend to keep them on the sideline or minimal screen time really, like Falcon or the other new characters in the last Avengers film --did you notice the tiny amount of screen time they all got?
This is due to the studio feeling they have to keep what is considered the main money draws on screen most of the time, in what I think are pointless little "setup" chat situations simply so they can be funny and have the usual little retorts back and forth, take shots at each other, that kind of silly stuff.
Which was quite funny the first time it happened in the first film but now that's over, it's been done and to repeat it in the sequel well... I find that very boring, distracting and a waste of my time and it certainly does not make for a very interesting film.
This is just my opinion of course but I think many see films with these and other short comings but fail to recognize exactly what it is they "don't like" you often hear "I didn't care for it" or "it just didn't do it for me" or whatever but no actual "specific point" of what "exactly" it was that caused that bad response, make sense?
I suspect studios suffer the same quandary in that they likely don't spend any or enough time at least--analyzing--to find out exactly what caused this film or that film to fail, which explains why they continue to make the same mistakes over and over...
'end rant...