That says more about the awful handling of the SM cast than it does about the benefits of having Satoshi around from my vantage point. If they were developed well, they wouldn't even need Satoshi as a foil.
Not precisely. As many mention in a previous thread, Ash works better bouncing off of other characters, and one of things I think degraded the use of character agency in previous series, aside from the blatant overuse of formula, was the characters never really merging their talents together in a clever way outside Team Rocket curbstomps that nearly anyone could do. A lot of people liked Ash and Clemont's dynamic in the earlier parts of XY for example since it was arguably the first truly developed 'brains and brawn' dynamic the series had, with Clemont's unique gadgets and smarts complimenting Ash's battling and physical prowess. When things swayed him out of limelight for Serena, who comparatively didn't have as many dynamics or niches that could play against other characters in standard stories, things started to become more generic and dumbed down again.
I see episodes of SM where the characters do get moments that work alright without Ash interfering, usually when it's their turn in centre stage, but those moments of team work and interaction do really help a great deal and add a bit extra life that wasn't often there before in previous series. A good way to make a story more character driven is thinking how one character's attributes comply with another around. Supporting roles can actually be fun and intuitive now.
Take the Ultra Guardian episodes for example, which are all about the companions merging their heads and special attributes together to make some sort of unified plan to stop an Ultra Beast that can't be stopped with simple battling. Sure there are a couple undeveloped companions that are straggling behind and not getting much involvement, but it's a good way of bringing out character and seeing how all the cast bounce off of each other and take affect on how things play out, which makes them more interesting and proactive than waiting for a COTD or Team Rocket to appear and make a simple plot to move things along for them, a formula I could argue is a far more blatant case of a protagonist failing to hold up a plot without another character or plot device.
Take Kiawe for example and notice that, despite getting as many central roles as the usual male companion his chemistry with Ash as a rival and mentor and more developed turns as the lancer/powerhouse of the squad allows him to stick out far more than some previous third wheels like Brock who were often restricted to exposition or running gags.