Recall that when the words "DEATH STAR" appeared on the screen within the first 60 seconds of the original Star Wars, audiences knew how the film would end. There was suspense in the Death Star Battle and the trench runs, there was universal audience ebulation when Luke made the perfect shot and the Death Star exploded. Yet everyone knew it was coming. Sure, the details may have been surprising (e.g., Han Solo coming to the rescue at the last moment), but the outcome was assured from the start.
With the prequels, the outcome was also known from the start ... it's the details that were disappointing (e.g., Anakin turning to the dark side easier than most of us might choose to cheat on a diet).
And foreknowlege of character fates or not ... I don't think the publics 'meh' with Hayden's Anakin, Natalie's Padme and (aside from the hearthrob factor) Ewan's Obi-Wan had anything to do with foreknowledge of their fates ... rather it was from poor writing, the poorest acting of any of their careers, nearly zero chemistry, and hardly any fun.
The public's delight with Mark's Luke, Carrie's Leah and Harrison's Han was because of their characters, their chemistry, their fun together and their seemingly great friendship. It had little to do with them being heroes or knowing they would come out on top. Audiences loved the the original trilogy's trio, and enjoyed their high-spirited adventures.
Audiences loathed the prequel trilogy's trio and deplored their decidedly low-spirit escapades.
The trilogies were yin and yang alright ... just maybe not in the way you posited earlier.
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