Maybe. Maybe not. Black Widow is, however, a vestige of the original Avengers. It's been a long time since Marvel introduced a new character that really took off. Dr Strange maybe, but he's not nearly as popular as the original core 3 of Cap, Iron Man, and Thor.
Spider-Man perhaps? We will see though. Spider-Man is almost damaged goods at this point. Captain Marvel is a loss. I'd say Black Panther would be a new foundational character, but obviously I don't think that can be the case now.
Thor has been recast. Cap is recast. Iron Man as well. It's an open question if audiences will accept these new actors in roles that were perfectly cast originally. I think people will struggle. The original actors just fit those roles to a T and have become synonymous with the characters themselves. I'm sure some might accuse me of not giving the new characters a chance. But if they decided to make a sequel to Blade with Anthony Hopkins instead of Wesley Snipes as the lead, I think people would be right to wonder how audiences would receive that.