I understand selling Griezmann from a financial perspective and if we receive a really good offer we won't turn that down but from a sporting viewpoint we could again be down to bare bones next season if he leaves. We'll have Messi (if AG leaves he'll surely stay, they won't both leave) and Depay (who will need to adapt but he knows Koeman's system so this shouldn't take too long) as the only dependable options.
Fati is returning after a longterm injury and we shouldn't rush him. Kun is old and has missed most of the last season so it's very dangerous to count on him as a starter as there were reports over the last few days. Hopefully Fati will fully recover and play up to his potential and Kun will stay fit but there is no guarantee for that. And behind them we'd only have Dembele who is well, just Dembele. Probably out until October then he could get reinjured again at any time. And even when playing he doesn't offer much.
Yes, Griezmann is not a prolific goalscorer we'd want (but we can't afford one at the moment) and he's surely overpaid but we can always count on him being available for 50+ games and have around 30 goal contributions (G+A) while also working his ass off defensively in every game. I know it's impossible for him to stay on his current wages but if he'd accept a 30-40% pay cut (in return for extending his contract for another year or two) I think we could keep him around if other amigos will also accept a pay cut and if we'll manage to offload players that are really of no use (Coutinho and Umtiti mostly) and those not counted on by Koeman eventhough I think they could be useful (Pjanić is the only one remaining after Trincao already left and Firpo being close to leaving as well). He has always expressed his wish to play in the MLS one day so even if on paper we'd extend his contract I still think he'd eventually move to America in 2023 or 2024 at the latest. But if Messi leaves in 2023 then Griezmann could also stay around as a bridge gap to younger players who will be brought to the club over the next few years to replace likes of Kun and Memphis.