I want for you to name one main character whose partner was a starter Pokémon for a specific generation who stuck around beyond that generation. The entire purpose of the starters existing in the cast is to act as an advertisement for that current generation and once that generation is over, they get shipped out of the team. Beyond that, they’re lucky if they ever get to do anything again.Go having a Cinderace isn’t as problematic as your trying to imply. For the most part they can easily have Go and the other stuff from Journeys stick around and just add the gen 9 region into the rotation.
As for the argument that Cinderace would overshadow the gen 9 starter, if you look at what’s being currently done with the gen 8 starters is their rotating the focus on the starters.
So it would be easy for them to have Goh catch a gen 9 starter and focus on it while Goh still has his Cinderace.
I mean heck if they keep with the world traveling format then Goh having a Cinderace isn’t really a problem.
Plus looking at his goal it’s really easy for that to be carried across more then one generation as there will always be new Pokémon.
When it comes to a Pokémon that is the character’s partner, they are integrally tied to that character and as such, once that advertisement period is over, the character goes with them. It happened with May, Dawn, Serena and Lana.
Now had they done Go like Brock where he caught Scorbunny but had already had a non-starter partner prior to that or even not tied him to having a partner then it would be more likely that he’d stay but dumped the starters like what happened with Marshtop. But from day one, they tied him to gen 8 by revealing his partner is a gen 8 starter. So come gen 9, he’ll move on from the cast since the gen 8 starters will be obsolete.