I don't think Ash will go away completely. As long as there's new games around the corner, the anime will get its hands on the pie just like with every other Pokémon game since.
However, I do think it's likely that it's the end of the weekly series that we know since 1997. What is likely going to be Gen 9 anime is a series of limited specials not unlike the Arceus specials. In fact, it was unusual for a game promotion to be given a dedicated limited series rather than be part of the main series. Yet, it covered everything you need to know about Hisui (without time travel) in the anime. And given the strains the main anime suffered to be a weekly series (lack of animators, long breaks), it is logical to assume that this would be the new approach.
Of course, such an approach means that Ash would not have any long-term goals. I doubt a mini-series would cover 8 Gyms without Ash rushing the whole thing. Ash being in the limited series would also limit any screen-time with his Pokémon team, either old or new.
In that regard, given that Ash building a team and reaching in a long term goal is the main appeal of Ash's story, the character arc of Ash would be finished with Leon, and Ash is semi-retired by the show. Thematically and technically, if the speculations are true, Ash's journey with us ends in Journeys. He and Pikachu will still go on to become Pokémon Masters, but our time with him would end. From time to time, he would show up to give us an update of his progress, but a lot of it would be left in the air and to the imagination.