Don't really see how Rotten Tomatoes was ever useful for "bashing" DC; most of their recent stuff has been rated "fresh," after all.
So far as Eternals getting an initial mixed reception, that's disappointing, but I'm not sure what the big deal is. Studios sometimes make subpar movies (even Pixar has had a few weak stretches). It is what it is. If Eternals is indeed bad, then this experiment didn't work. It's not like Marvel Studios has anything to prove as to why they're the king of the superhero movie or anything.