Ash and TR have marketability yet but they also can't go on forever with him else it becomes stale. Considering the direction of the anime I won't be surprised if gen 10 anime does have a new protagonist, a new story. They could still have another Pikachu on the cast. I don't think they will be afraid to do it. And yes, TR will probably be retired if ash's journey comes to an end. So yeah, I personally believe ash's journey will someday have to end, and gen 10 is a nice spot for that to happen with Gen 8 ending with him being the strongest trainer and Gen 9 further reflecting on that. Again, my theory
Becoming stale? Dude, they already run for like 22-23 years, if that's not stale, I don't know what it is XD. They run with James Bond for like 60 years. Godzilla with also 60-70 years, Mickey Mouse for 90 years. Tom and Jerry etc etc etc, and yet they're not stale. If you're true Ash or TR fan, they wouldn't be stale. For me Ash is not stale. If he becomes Pokemon Master and anime continues with him then I can say: Yeah, there's nothing beyond that for Ash, it's stale "but it makes no sense for me that anime continues after Ash becomes Pokemon Master". Not to mention that for writers : marketing = money >> stale.
They could have another Pikachu yeah but that would be pretty bad or "stale", look this is ???(insert protag name) his first Pokemon is ... Pikachu! I find it bad. In this case I would've liked more if it's Alain like guy instead of being carbon copy of Ash.
Well, it's a fact that Ash's journey will end. Like everything ends, it's just matter of time. But based on all things considered and how things go: Ash's journey will finish and anime will end. Anime will end when writers stop making money and that would mean Pokemon is in decline. Again; look at Mickey Mouse, 90 years and still running and there are still Mickey's cartoons since Disney is making money. Don't be surprised if you're like 80 and Ash's still on his journey, yeah that sound bizzare but again: Tom and Jerry, Mickey Mouse, James Bond ,Godzilla etc. Not to mention that's it would be too risky to remove Ash,TR and Pikachu because they generate money. I mean look at Yugioh; they change protags and later series don't preform very well. And last thing no one knows what is Pokemon Master. Writers can literally pull anything about of fart machine lol... Maybe they would say to become PM you need to catch all Pokemon, basically Ash does Go's job if Go leaves.Maybe it's that, maybe it's that.. or maybe it's all those things: get all Pokemon and be the strongest idk.