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That’s what I think will happen too.
Even in Toy Story 3 the conflict of “your child will grow up and won’t need you” was postponed with Andy giving the toys to Bonnie. She’s going to get older still and give up the toys and it’s just another delay of the inevitable and heartache will restart once again.
Lightning retiring after having one last comeback by beating Jackson would be beautiful, heartbreaking, but hopeful ending that would resolve his internal conflict and having him leave on his own terms. He’d see it as a choice he made, that Doc didn’t have. Doc said there was a whole lot left in him after his crash that he never had the chance to show it, the crash left him bitter and angry at the racing world and he wanted nothing more than to hide and never hear of it again. Lightning doesn’t want to be that, he doesn’t want to be kicked out of the sport he loves, he doesn’t want to become resentful of it when he stops. He wants the choice of retiring, not the fear. Leaving something that you love doing, knowing that you accomplished a lot and made it your choice is a satisfying end.
Him retiring and maybe even becoming Cruz’s crew chief would be his character arc coming full circle, becoming the Doc to a new generation, to Cruz.
Hear me out: yes, they can do a movie where he’s falling behind new tech, refuses to give up, trains with Cruz, gets revamped, goes into another race and shows those whippersnappers who’s boss. And it could be fun and entertaining and have good characters, and have a message that just because you’re old doesn’t mean you’re out! However, the main emotional conflict for Lightning is unresolved. He still has to let go and retire at some point. There will always be new tech. So really, the main conflict is only postponed.
OR
They could do an ending where he does train, he does get new tech. He could potentially beat Jackson Storm. But then he sees what a promising racer Cruz is (isn’t she supposed to be an aspiring racer?). And he starts to reminisce about his career, and he does realize that he’s had a really great one that will leave a legacy no matter what. And that there’s things he can potentially do with his life (and his wife/girlfriend/whatever) after he’s done racing; his life isn’t over just because he retires. So yeah, maybe he does go in and beats Jackson Storm, just to show that he can and to leave on a high note a la Michael Phelps. But then he retires on his own terms, opening a space for Cruz to enter the circuit, because now he realizes that his legacy can last through the new athletes (again, Michael Phelps–he’s getting beaten by guys who idolized him as kids now and he’s cool as long as they aren’t asses about it). It would close the series, like Toy Story 3, with a message of letting go, letting a younger generation take over, and entering a new chapter of your life. In my opinion, a much stronger and more complex film.