Guendouzi is not a tool, he's young and a bit of a hothead. Vieira was a better player obviously but used to get in trouble for no reason all the time, then let's not forget him taking the piss the summer before we won the double with him even going for his team mates and his manager by name. Didn't end up all that bad with our captain did it.
Oft if you think it's all circumstantial and luck then you really are the ideal customer, sorry to say it. Liverpool sold their garbage at inflated prices long before they won anything, can't be bothered to post a full list but how Many Ibes Solankes Brewsters have they pushed out for fuck off money. Chelsea probably even worse, Costa Morata as chewed up as they could get and they made 100m+. Now they're selling some academy kid to skint Milan for like 30m.
Im not even a sales man but the point is you make your market yourself and find suitors for your players, not sit on your thumbs waiting for a bloke to burrow through your goods for a fee reasonable to him. We all agree that Guendouzi could've been sold for at least double that some 18 to 24 months ago, what's keeping us from getting Arteta to publicly reinstate him even if only for show, possibly extend his contract with a release clause, drum up some interest using some agents here or there. Disregarding obviously that we could use him ourselves which Arteta seems to rule out. Literally every other option is better than letting him go like this.