I doubt that all of the heavy hitters will leave the team. Back in the early 90s, the X-Men took over as Marvel's biggest franchise since first Spider-Man, Namor, Quicksilver, Hank Pym, Wasp and Moon Knight left the Avengers titles, then after Galactic Storm, Captain America, Thor, She-Hulk and Iron Man were no longer Avengers.
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Point being. Bendis was able to get rid of the characters that Chuck Austen crapped all over because there were still enough heavy hitters left from Busiek and Johns to play around with and when they were introduced, they were back to being well-written while Bendis brought Spider-Man back onto the team and introduced Luke Cage, Iron Fist and Wolverine while slowly reintroducing characters from the team's past over nearly a decade.
The Avengers need familiar characters. Without them, they aren't Earth's Mightiest Heroes, they're just Marvel's leftovers. If Marvel were to ditch everyone from Phase 1, we'd only have Ms Marvel, Black Panther and Blade as the only A-listers and Scarlet Witch, Quicksilver and Vision as supporting cast since all the street level heroes will be Defenders instead. So think about that. Instead of allowing the franchise to grow, Marvel will be focusing on only three stand-alone properties.
I'd much rather see the Avengers get broken into two teams after Phase 3 to deal with all the new blood.
Mighty Avengers: Captain America, Quicksilver, Ant-Man, Iron Fist, Thor, Falcon, Luke Cage, Jessica Jones, Darkhawk, Crystal, Black Panther, Namor, Winter Soldier and if Marvel can get the rights back Spider-Man
West Coast Avengers: Iron Man, Hulk, Ms Marvel, Scarlet Witch, Hawkeye, Wasp, War Machine, Tigra, Black Widow, Daredevil, Moon Knight, Sersi, Medusa, Blade and Vision