What happens with the Buffalo Bills' offense if James Cook leaves after 2025?
To answer this question requires some thought about two things.
The first is who replaces Cook in 2026, and the second is what the Bills' offensive line potentially looks like in 2026?
This question was recently posed to Sports Illustrated senior writer Albert Breer, who took a shot at answering, and here are a couple of his points.
"Cook's a stick of dynamite who can do a little bit of everything. Davis is a mudder, a tough guy with a fire hydrant build who can grind out the tough yards.", said Breer, who later added, “I wouldn't close the book on a Cook deal yet. The sides are nowhere near getting anything done, but there are still some checkpoints in the calendar that could certainly get one side to try to get a deal done.”

It's unclear what Breer means by "some checkpoints" but perhaps he is referring to training camp or the start of the regular season. Also, his reference to "one side" brings into question which side he is referring to. Regardless, both sides will need to come to an agreement, or compromise, but if either side digs in, then we could be looking at a Bills squad without Cook in 2026.
Breer's response to this answer seems to indicate he does not believe that Davis can carry the load by himself, while also saying the same about Cook. The thing is, the Bills' running game is far more about their offensive line than the guys who carry the rock. There have been players of similar styles to both Cook and Davis that more than carried the load for their respective teams. If you have an elite line blocking up front, your running game will be fine.

Besides, we've seen the Bills for the last several years, and they don't subscribe to the workhorse back ideology, but not exactly a committee approach either. The Bills are somewhere in between.
The Bills could very well roll with Davis as the lead back, with Ty Johnson and Frank Gore Jr (or a 2026 draft pick) to carry the load at times.