The core Avengers have been the same for decades: Iron Man, Captain America, Thor, Wasp, Hawkeye, and the like. The B Team can be cycled in and out, but the core and especially the Big 3 are the rocks Marvel needs to build their foundation on. Those are the Avengers. The entire concept is about the biggest named superheros in the Marvel Universe joining forces to save the world. When the team is the likes of Squirrel Girl, Big Bertha, Mach-5, Sentry, and Speedball, it won't matter if they are called the Avengers, they won't be THE Avengers. Marvel absolutely needs the popular, iconic members to be on the team.
Agreed, I couldn't say it any better!