Guess I'm ignoring the Avengers films from here on out
I don't know why people say this. There will still be other heroes.
Honestly, we're not even sure the Big Three will be gone either. Chris Hemsworth has renewed interest in Thor thanks to Ragnarok so he might stick around. RDJ brings in too much money for Marvel to not at least keep him around for team-up films or cameos. The only one who really has a serious shot of ending is Chris Evans' Captain America. And there's a strong chance Bucky/Winter Soldier will replace him.