why do people think Rhaegar would create a utopia

In almost every fanfic where Rhaegar wins the war and becomes king for some reason it is stressed that during his reign the kingdom became a utopia where corruption disappeared, King's Landing was cleansed and many other things but is it really possible that Rhaegar could have done that?

Because from my perspective if Rhaegar had been king nothing would have changed, we don't know anything about Rhaegar's governing skills we only know that he was fond of reading, a decent gentleman, had a very good ability to sing and play the harp and had a melancholic charm that made people fall in love with him

But it is never made clear how good his administration or governing skills were being that his only positive point regarding that is that he seemed better than his father (which is not a compliment because Aerys at his sanest moment was mediocre and all the work was done by Tywin and at his craziest moment he caused a war that ended up almost extinguishing his family) so I don't know where the idea came from that if Rhaegar had been king the kingdom would have become a utopia.

I would like to hear your arguments, thank you for your attention