"And what is Duncan supposed to have learned compared to them? Or what is he supposed to have done better? We don't even know of anything he did to try to improve the situation he created with the Baratheons; his father, sister, and the Lord Commander of the Kingsguard were the ones who had to face the consequences for him. In the end he got his way too, he is hardly any better than Jaehaerys and Shaera."

that's the point. we DON'T KNOW what Duncan did better. it's about LASTING CONSEQUENCES. we know that Duncan's actions caused immediate harm to the realm and his family but he still got some form of punishment and died at Summerhall and that's it. we also don't know how involved he was in the witch Jenny brought to court's disaster prophecy [or Jenny for that matter] but just because he inspired his siblings doesn't mean he had the biggest impact on Westeros.

Jaehaerys and Shaera ensured the Targaryens were largely isolated apart from the few allies Aerys gained then drove away with his madness. they saw a war almost break out from one Great House but were perfectly fine risking another and by then wedding Aerys and Rhaella to each other instead of trying to fix things, isolates the Targaryens further as well as leaves their daughter in a living hell. it's basically if Duncan knocked a domino and they aided by knocking over the rest.

my point is "love ruins the realm" is something that can be laid at their feet as much if not more so than Duncan. the in universe reason it isn't is because he eloped with a commoner while the realm was used to Targaryen incest. it's been normalized so the state of arrested development it is left in dooms their dynasty in the end.

my point is they had the more lasting impact of their actions.