This is how I see it. We're in crisis, we need reinforcements. A guy like Thuram (let's say hypothetically, Diego) says sure I'll play for Juve. Fine, Diego turns out to be great, just what we needed. A few years later he leaves. The fans suddenly hate him. Oh you ungrateful mercenary, how could you yadayada. The thing is though, at the time Diego was signing that contract he took no vows to worship the club forever. What's more, if he were explicit about it and he said I have no intention of staying here a moment longer than to further my career, and you were a supporter of a club that desperately needed him, there's no doubt in my mind you would have struck that deal anyway.