I suspect the real reason for not having every companion show up at once was more production related than creative. However, I should point out Ash has never really cared if his friends are in the crowd watching him, and would probably rather they do their own thing rather than drop everything to come cheer for him. His reaction to Dawn and Chloe being there was basically "oh, cool, thanks".
Same for the post-win party. Yeah, it'd be nice to have everyone there, but by the time Ash gets home and passes on the trophy, he's already thought about his next adventure. He's not one to linger on successes anymore.
I feel like BW kinda got slept on for trying to breathe some new life into the show after a lot of its main components just slowly got more and more automated and repetitive. Sure, it definitely didn't execute them all well, but it was the first time in a long while they were TRYING. Stuff like more ongoing plots besides just the standard protagonist tornament and region villain showdown, giving the male companion more to do, having episodes that don't need Team Rocket as a crutch while simultaneously giving the TRio more ongoing stuff to do, focusing on the protagonists' personalities and dynamics as a driving force of the story over just their progression.
I actually thought it was a real downgrade that XY just sorta trashed a lot of those things just because the trial and error didn't work out perfect. SM brought them back and tried to refine on them, which I did enjoy a lot, though even then I think there's a lot of potential BW offered that hasn't yet been seized on properly.
Whatever potential BW had was tarnished by utterly bizarre creative decisions that unfortunately established the show's poor legacy. For example, laying solid foundations for Ash's rivalry with Trip, only to ignore the rivalry for large chunks of the show, have Trip care more about Alder than Ash, and then wrap up the whole thing in a one vs one battle at the start of the Unova League. Only to then have Ash lose in a 6v5 against someone stupider than him, thanks to an ass-pull evolution. Apart from the strain on the audience's credulity, none of it contributed to the show's themes or characters.
I have more time for BW than most, but there's a reason it's received so harshly. Whatever criticism I can have for other seasons, BW stands out as the one with the most inconsistent direction, from the way it's structured (having a filler arc AFTER the league and Episode N), to how it handled its cast and story.