Switching Ash's and Goh's goals would never work, and regardless how good a story the Anime tried to throw out. The writers simply need to do better with balancing things between Ash and Goh.
No offense to the writers and the goals they have assigned to Goh, but his goal of catching every Pokemon, once upon a time, that was an intriguing idea, back in Gen 2 or Gen 3, but now, it's just silly, because we know there's always going to be more Pokemon, the idea of catching them all, is utterly pointless, especially since it means that all the years of complaining about Ash and the other traveling partners not developing their Pokemon properly, all of that falls way short of how poor a job will ever be done with the vast majority of Goh's Pokemon. His goal of catching Mew, while there's ways it could be made interesting, the route the writers are taking to work towards the goal is so vague that it lacks any real meaning or significance, and just say for the sake of argument he catches Mew, what next, does some other mythical legendary become the new end game capture for him?
Ash's goal needs more focus time, period, he is the main character still, and likely to continue into the future, so the amount of time devoted to Goh as opposed to Ash as the main character is utterly ridiculous. And people think Ash's current Pokemon team are featured poorly now, just imagine if he gained Goh's goal to just capture everything willy nilly. Ash's goal needs to start featuring more high profile opponents similar to how gym leaders were the main opponents in past series. Currently it's a hodge podge mix of random and sporadic nonsense. It would have made more sense if there was a period of time set in Kanto, where Ash had to face the Kanto gym leaders, but they are using more competitive teams then they use in gym matches. And then move on to Johto and do the same with the Johto gym leaders, and then Hoenn, and then Sinnoh, and then Unova, and then Kalos, and then Alola which needs to have actual gym leaders introduced since that region and it's league are established after Ash's victory having been it's first year supposedly, and then Galar. Then after that a series that features Ash facing each regions Elite 4 members one at a time. And then the next series features him facing each regions Elite 4 Champions. Maybe there could be as a bonus thrown in occasionally feature past trainers that were shown to have gone on to win the various regional Leagues, since they are all in theory on Ash's level. Point simply being, Ash's goal now really is too vague, and the structure of his opponents to sporadic and who he faces there's no rhyme or reason as to why he faces them when he does.
