The MCU has already thrived with "The B-Team", they'll be fine without Cap, Iron man, etc. Pre 2008, Iron Man was a B-Tier character at best when it came to pop culture, making a movie about him was just as odd and probably more risky an idea as making movies about the Guardians of the galaxy or Ant-Man were. Cap and Thor were just as unknown in the mainstream as IM if not moreso in Thor's case.
The MCU was built mostly with characters that people who weren't comics fans had barely heard of (minus Hulk, obviously). I'd say that the MCU has already built Black Panther up to the level of the original Avengers when it comes to how excited people are to see him, and they have a huge untapped roster, including the FF and the X-Men.
The MCU is nowhere near fading away. Its got hot properties, and a lot of stuff till ready to be used. They'll continue to thrive, even without the old school Avengers (or Spider-Man, for that matter).
Sony, on the other hand, is shit. I doubt the venom sequel will get to coast to victory like the mediocre first movie did, and Sony hasn't made a good live action spider-man film by themselves since Spider-Man 2. Since pretty much no one, not even hardcore comic fans, give much of a shit about Sony's 10,000 spin off films (did anyone actualy want a Morbius solo movie? or the potential solo stuff with Kraven, Silver Sable, etc?), I'd say Sony is definitely going to end up worse without Marvel.