If TR leaves at the end of this gen, Ash will be the only character left. With everyone else gone, you might as well do a REAL reboot for next gen.So? It's still totally fresh even if they sometimes mention past events... and even if you'd replace Ash with Hilbert in BW, I honestly doubt ANYTHING would be different other than the face of Ash being now the face of Hilbert
Also, funny to hear that from you, since you always insist on saying BW is a "reboot"
That's not to say Ash can't appear at all. I could see him being something of a recurring champion like Cynthia who only appears every 25-30 episodes or so to challenge the next protagonist.