Intentional mediocrity is incredibly hard to achieve. Smart people who can create something that's perfectly mediocre and can be consumed by the masses are the true geniuses of this world.
https://en.wikipedia.org/wiki/James_Cameron
Avatar and Titanic are stunningly mediocre films. But look at the box office numbers.
Avatar and Titanic presumably aspired to be excellent films as well. The visual effects that went into each were nuts, and Titanic's production values were quite good.
Why legendary directors sometimes end up making progressively worse films later in life is a rather complex question.
I'll start: Spielberg.
I would argue with this, though: "Avatar and Titanic presumably aspired to be excellent films as well."
No, I don't think they did. I think by now Cameron had internalized the screenwriter's gospel of character arcs and what audiences (presumably) want to see, and regurgitated it. "Excellent" to him meant "what people will pay for."
I suppose it's fear of your movie gross failing to exceed your production budget, and you retire as a legendary director anyways. Ah, that doesn't seem too bad. Well worth the risk. But, no.
Chuck Lorre (creator of Two and a Half Men and Big Bang Theory) comes to mind.