I disagree. I could easily see him leaving Cinderace at the lab to be like Ash’s Bulbasaur once he gets some of the newer Pokémon or a Pokémon he rotates in for harder catches. Everytime a Pokémon on the main cast is brought over from the previous series, it gets minimal focus and eventually fades into the background if it hasn’t been traded/released already. I don’t think it has to stay permanently if Goh continues to travel with Ash. Pikachu is the mascot of the entire franchise so obviously it was going to stay. Cinderace is just another flavor of the month starter.
Again, though, Cinderace is Goh's very first Pokémon
. That's quite a bit different from Ash's Bulbasaur. Very rarely, if ever, do characters in the anime take their very first Pokémon off of their teams. Sometimes, they do. But not very often. Even to begin with, Ash, Misty, and Brock (but mostly Ash and Brock) are the only constants that carried over between generations. Accordingly, they're the only frame of reference we have for companions carry over and trading out the old generation for the new.
Ash always keeps Pikachu, as established, but even back in Battle Frontier, he brought back Phanpy and evolved it into Donphan! Donphan wasn't getting any special treatment in Gen 3, and HGSS was still too far away for promoting a Johto Pokémon to make any sense. In Black and White, he brought back Charizard and Iris got Dragonite. Neither of those Pokémon were really relevant in Gen 5. You can't even argue that it made sense because Charizard was getting a Mega Evolution soon, because Ash didn't bring Charizard to Kalos. And now, in Journeys, Ash only has two Galar originals, Sirfetch'd and Dracovish. And Gengar can Gigantimax Lucario and its potential Mega Evolution aren't especially relevant anymore either. And Dragonite still gets nothing special in the games either.
Misty kept using Staryu and Psyduck a decent amount, and barely used Politoad at all in Johto. Sure, she used Corsola, but that's because even ignoring marketing, her Corsola's Recover was shown to make it look OP in the anime. When she eventually returned in the anime in Gen 3, her new addition was Gyarados, and this is years before Gyarados would later get a Mega Evolution. And we also never learn what Pokémon was her first, so if it was Staryu, she kept THAT the entire time.
And as for Brock, we never learn his first Pokémon either. But it took until Gen 4 for him to stop bringing and stop using Pokémon from previous generations. Heck, he left Ludicolo at the Pewter Gym partway through Gen 3, even though keeping THAT ball of energy around during ALL of Gen 3 would have made more sense.
Even if Goh carries over to Gen 9 (I personally don't WANT him to carry over, but if he does, he does), I don't see Cinderace going away or being given any less focus. Because unless it's unclear what their first Pokémon was, characters tend to always keep their firsts on their person. And the OG trio already proved that marketing doesn't always reflect what they'll keep, ditch, or bring back.