Endgame could well be the last Avengers movie. Doesn’t mean there isn’t another movie with a group but seems likely it won’t be the Avengers. The last movie didn’t really feature the Avengers as such as they were spread out and be coordinated actions. This will be their big coming together, potentially as a swan song. There are other teams to bring together for Marvel now, so we don’t need to have them.
The Avengers are increasingly seen as the big must see movies with smaller ones, whereas you could turn every Avengers movie into must sees by overlapping the universe more often in solo movies. Spider-Man, the Hulk, Iron Man and the Fantastic Four are often in the same comics, even if it’s just for one of them. After endgame there is an exciting opportunity to move more towards this than before.
It’ll take another big bad the size of thanos years to build up anyway, so you won’t necessarily need all of them in one movie, and with the additions to the current line up it won’t be the best idea to try and get them all in one as it becomes an unwieldy behemoth. How would you even start getting the whole cast, plus the fantastic four; plus the x-men into one movie!
Whilst they need the central hero for a movie to drive their story, adding more logical decisions like a superhero turning up for a fight because they’ve seen the danger to support the central hero would only add to the stories I think.