To me there is a freedom and opportunity in how one answers those questions. With Anakin and the original trilogy, all we know is that he was seduced by Palpatine and fell to the dark side, with some general comments about his friendship with Kenobi and status as a pilot. A good story can take those very base elements and craft an interesting examination of characters and settings with it. The story of Vader becomes what makes a good man go bad, same as what makes a Republic turn into an Empire.
Look at something like Godfather Part II, which while only part prequel is brilliant at giving Vito Corleone a detailed history which defines not only his character, but also serves as an effective contrast to that of Michael’s. The base concepts of what Episodes 1-3 wanted to do are not flawed in and of themselves; seeing where Anakin zigged where Luke zagged, getting an idea of what the Jedi were prior to the purge, and providing a window into what conditions allowed Palpatine to grab power. Their failure lies in the execution and answers to those questions being illogical, trite, or unrelatable to viewers.
Yes, the original films benefited from mystery and imaginative world building, but it also spoke to an audience because they connected to the characters. Leia was a strong (literal) rebel fighting for what she believed in, Luke a rural kid with big dreams wanting to make something of himself, Han the cynical wiseass with a hidden compassionate side to him. The prequels didn’t work to a great degree because they simply couldn’t create engaging people the audience was capable of getting into. Anakin moves from darling kid, to bratty teen, to murderous Sith, without ever feeling a person, doubly so with Padme who’s basically the generically good love interest to him and who exists to marry him, give him some kids, and get out of the way via dying.
Lucas was guilty of over-explanation with the prequels to a point, but the idea that no explanation of Vader and others would suffice strikes me as a copout, and ignores that many pieces of media and television have detailed the backstories of major and enigmatic characters, and still made it work because the meat lied in the journey not the destination.