I dunno, Prime; do you really believe we had to see him grow up on screen in order for us to feel the weight of his tragedy? We didn't need to see a character like Michael Corleone as a kid squealing "Yippee!" - or as a teen courting Kay by letting her know about his displeasure with sand for getting in his butt crack - in order to believe that he was a good, decent, and normal man before selling his soul for revenge, greed, and power.
In two movies, Coppola accomplished with Michael what his buddy George had six movies to do with Anakin/Vader. Two movies (two of the greatest ever). One movie did all that was needed to portray Michael as an upstanding man who Kay had fallen in love with despite her disdain for the family business. The other movie gave us a profound view of that man transforming all the way to the point of having his own brother killed.
We had already seen Anakin die to save his son and prove that he was indeed the "good friend" and honorable man that Kenobi described him as in ANH. I don't agree with the argument that we wouldn't be able to feel the tragedy of his turn if we didn't see him grow up. And as for needing him to be whiny, I think that's nonsense, tbh.
It's fine for Luke to be a whiny 20-year-old because we don't need to believe that Luke ends up occupying a Vader suit and giving off a natural and effortless commanding/lordly persona. If you only showed me clips of OT Vader with no context and then showed me clips of OT Luke and told me he grows up to be that character, I'd laugh at that too. It just wouldn't work in terms of intuitive congruency. The man in the Vader suit is calculated, cerebral, and in total control. For me, PT Anakin just simply does not work as a believable candidate to transition into that OT Vader.
A good-guy Anakin should still be able to make you pee yourself a little when he gets angry. He needs to be someone we can take seriously. There are plenty of people in this world who give off that vibe and are still good, kind, and decent individuals who are easy to sympathize with. But turning evil doesn't magically imbue someone with those qualities; you either have it or you don't.