The truth is even doing a Gym challenge isn't that interesting, because it would be a carbon copy of Ash's goal/role. That's why they didn't do it, besides having them battle Gym leaders in a training arc which wasn't for badges.
I don't think that's why they didn't do it. They can make a Gym challenge without making the main character a carbon copy of Ash. Both May and Dawn had the same goal, but both their personalities and Contest arcs were still different. I don't think it would be that hard to do, especially when there has been a long break of a traditional Gym badge quest in the anime. The Adventures manga series has done multiple Gym badge journeys with characters that have different personalities and goals. Granted, that is a different medium with different writers involved, but my point is that it is more than doable to make a Gym challenge different from Ash.
Ash's story should of ended long ago, even SM could have played out exactly as it did with the same cast just without Ash/TR coming back. Just have some new kid meet the Alola crew, Kukui would have the father dynamic with him, and everyone else like Kiawe/Lana/Mallow/Lillie/Sophocles, etc. could have all been the same as they were. SM would have worked just fine as a reboot without Ash returning (of course I'd have him with the Kalos league to give him a proper sendoff, etc). And Journey's would just be something else entirely since Ash would have been gone, could have had Go and someone else I guess since Pokemon Go heavily influenced that series.
I still highly disagree that SM would have played out the same without Ash. Ash's personality in SM wasn't so different that he could have been replaced with a different character. The father/son dynamic between Ash and Professor Kukui was memorable in large part because Ash never had that kind of dynamic before. Putting in a new character in Ash's place wouldn't have made that connection or their battle at the end of the series nearly as memorable as it. Ash's dynamic with the other characters wouldn't have been the same with a new lead either. I just don't see Ash as being so easily replaceable in SM given his personality, role in the overall storyline and dynamic with most of the cast.
I love Goh, but I don't think he could have been the main character of Journeys either. He was designed to be a co-lead with Ash and their dynamic was a big part of the series. That couldn't be easily replaced with just another lead character. Both their dynamic and most likely Goh's personality/story would have been pretty different if he was design to bounce off another character. I still don't understand why people think that Pokemon Go heavily influenced Journeys. That isn't to say that there isn't any connection to Pokemon Go, but it's all pretty surface level stuff. The only connections to Pokemon Go I can recall were Goh's name, his catching method, I think the concept of Max Raid battles since they were more similar to Raid battles in Pokemon Go and Pokemon eggs having the generic designs. That's far from being heavily influenced by Pokemon Go. Goh's first Pokemon was Scorbunny and they didn't start adding in more Pokemon from the Galar region until last year, well after Journeys wrapped up. I just think fans really exaggerate how much influence or promotion Pokemon Go had in Journeys.
Even XY if you take away the animation boost was just ok, it had its good points but looking back it was just more of the same from AG/DP without much to stand out to make it much different. People only latched onto it as hard as they did because Ash looked older/competent, people liked shipping with Serena, and people thought he was going to win the league. Bonnie/Clemont were great though at least. Hell Ash's lack of a rival till halfway through it really made it feel like AG again too. I can see why the younger generation liked it so much though. It was a fine 7/10 series boosted by really good animation for Pokemon standards and the people who liked the shipping and/or AshGreninja.
I disagree that there wasn't too much to make XY stand out from AG/DP. The group dynamic was still distinctly different, the Gym battles were pretty solid, Ash's team had some good moments and both Serena and Clemont had some good character development. The Team Flare arc was also a huge highlight of the series. It had some great emotional moments, every character in the main cast got a chance to shine and it was a satisfying payoff to the Team Flare storyline. I still think that Team Galatic were overall handled better in DP, in large part to how they were a major storyline over the course of the series instead of just for one year, but the Team Flare arc was still really good. I wouldn't dismiss all of its praise to just people thinking that Ash looking more competent, liking his pairing with Serena and thinking he was going to win the Kalos Leagues. Those were factors for some fans for sure, but I think that dismisses a lot of what works in XY. I wouldn't even say that it was just younger fans who liked it since plenty of long time fans enjoyed it too.
I remember saying since 2010 Ash's story should of wrapped up. While I did get my wishes, I wish it didn't have to take yet another 13 years till 2023. And I still didn't get the TR/Giovanni finale yet, so I suspect we'll get that if/when Ash gets a movie/special down the road.
I wish that it didn't happen myself honestly. I just never felt like Ash needed to be replaced and while I'm sure that I'll enjoy Horizons whenever I get around to it, I doubt that the series will change my mind in that regard. I don't really see the need for a Team Rocket/Giovanni finale. I don't think that they'd want to get rid of the Team Rocket organization for at least two reasons. They're the most iconic evil team in the franchise, so this gives them a way to introduce new Team Rocket members into the anime whenever they want to. The Team Rocket trio love being in Team Rocket, so even with them not being in the cast, they probably would want it so that they could still be in Team Rocket. I'm sure that we'll get something for Ash eventually, but I just don't see the need for a final showdown between Ash and Team Rocket when that was never really a goal in the anime.