I dunno, I think outside of the anime just being nothing more than a long-ass advertisement for the games, Ash was always meant to show a gradual progression in skill. Nostalgia blinded fans for so long that the first season is still highly regarded, but Ash was fucking terrible when he started out. He never gave up, however, and in his determination he always managed to find a way to defeat his opponent in a rematch. Some of the tactics are questionable (such as when he super-powered Pikachu to face Brock a second time, and even then he only lucked out because the sprinklers got set off), and he didn't always earn his badges because Team Rocket intervened, but his Vermilion and Volcano badges were ones he earned fairly, and that's why those victories are still well-remembered. Ash was a cocky kid, however, and him losing the Indigo League may have come as a shocker for us kids at the time, but he had never put in the effort to train like he was supposed to. The seasons that followed were where he was shaping up to become a better trainer, although he still had to find humility such as when he let Charizard go for a while to train because he knew he wasn't skilled enough at that time to bring out Charizard's potential. Some fans take issue with that because "Bwaaaah he gave up his best Pokemon, what a moron!", but that was a smart move to make on Ash's part.
So as a result of him pushing himself to get better, he got recognition in-series, even from high-ranking trainers like the Elite Four members. Scott offering him a spot as a Frontier Brain may-or-may-not have been earned and he turned it down anyway, but the position is still open to him should he ever change his mind (although the writers might've forgotten this for all we know). Filler season or not, that's a big deal and shows just how far he had come from when he first started out. The anime's poorly-written, there's no questioning it, but it's still fascinating to see Ash progress in such a way that he could land himself such a respectful rank, and at a young age, too.
It's too bad that the team dropped the ball with Best Wishes and fucked all of that character progression up for whatever reason. I do think they were biting off more than they could chew by this point and needed some excuse to do a "soft reboot" or whatever, but they still kept making callbacks to previous seasons and the like which made everything confusing. Still, Ash sooner or later was going to win a League. Given that Sun & Moon appeared to have made the effort to change up the status quo, I don't think it's really all that shocking he won in Alola. Also, Alola's island challenges basically are just the Orange League but better fleshed out and with a recently implemented!!! conference.