I think he still has a lot to offer us. Given his fitness issues he's no longer a nailed-on starter for us - but then, lots of players aren't. Pep's commitment to rotation means it's not unusual or any great hardship to have yet another player who can't or won't play every minute of every match.
We have to keep the balance right, as he's a player who needs time on the pitch to stay sharp, but his fitness can't keep up with playing all the time.
I'd like to see him carefully managed with involvement in on average 1 match per week, which match it is being decided on the basis of where we think he can most benefit the team, combined with how quickly he can recover after his last involvement.
And whether that means playing in his traditional midfield role or further forward, we also need to start getting some playing time in the middle for one or more additional players (presumably Foden and/or Grealish).
Our usual formation needs 2 players there, we currently have 3 fantastic players to cover those positions, but only Bernardo has the stamina to play all of every match. If there was a significant injury to him, KDB or Gundo, one of the latter two would have to increase playing time (if Bernardo was injured, they both would) and I think there's a real risk they'd struggle to do it at the level and consistency needed.
I think we have the skills in the squad to share that burden, just need to make sure we build the experience too.
But I wouldn't sell Kev unless he personally wanted to go. I think he still has match-winning moments to come, and a role to play in the development of his successor/s.
His contract runs to 2025. Depending on how his ankle holds up, I can see him retiring then or before. We've got space in our squad to bring in other players without getting rid of him. I say keep him for as long as he's offering us anything and his presence isn't hindering the squad.