I'm still thinking this through, but just to throw it out there:
Throughout OP, Oda is a master of writing morally gray characters. In Wano especially, it seems like we've been meant to root for "good guys" who aren't quite 100% great guys, as well as giving "bad guys" more sympathy than they sometimes deserve.
Remember the fan backlash when Komurasaki was revealed as an oiran? People hated her just because she was a prostitute, even though she was more like a high-class escort. People treated her as suspect just because she was an oiran and said she couldn't be trusted. Some of the fanbase is still hung up on that fact!
Then we got Yasuie, a totally heroic character who, uh, sold his daughter to be a prostitute companion. But we still think of him as a "good guy." We've also vacillated on Kyoshiro, Shutenmaru, Shinobu, Speed, and a few other gray characters, which fits Oda's style of course. We've done 180s on Drake and Killer as Oda showed us a different side of them.
But to make Oden's story more powerful, Oda had to make him suffer more, and to be more distasteful. What could be worse in the OP world than turning your back on your country? Vivi says hello! Turning your back on your own elderly father? Just because the adventure of the sea was more exciting? How selfish! Oda went as far as to drag our hearts through the mud by showing us how Toki suffered at sea, not to mention how even Roger called Oden names. So clearly, we're supposed to think Oden is a flawed (not evil, but not ideal) person.
And what tough-guy trope is ready to answer our concerns? The non-physical trial. Of course every literary tough guy can withstand physical pain, but psychological pain is the typical step-up in storytelling. Think of a POW who bides his time in the enemy camp, knowing that someday justice will be served, or even think of Señor Pink, whose "humiliating" appearance only proved his toughness. A truly strong character would welcome this psychological test as a proof of their strength.
But we know that Oden didn't make it. He DID pass the test of humiliation, but he didn't get his reward. I don't think Oda means to present Oden as a perfect hero or a Chad. He is showing us a flawed person who was the first to test the boundaries of his country. He was always presented as a rule-tester and a rebel. As far as morally gray OP characters go, Oda took this one to an extreme:
1) to punish Oden appropriately so that the readers feel satisfied
2) to make the current-day battle more meaningful
3) to make the readers feel the same frustration as the citizens of Wano. We're ready for the Strawhats to fix this shit, and so are they.
Like I said, I'm still working on these thoughts. What do you think?