This is what I think is mental. It's clear that we're on very different ends of the spectrum on this so I'm going to let it lie.
Albeit I'll make one observation in regards to your final question. I think football supporters go to watch their team because they support the club. They support what it's about and what it represents to them. Players, however, simply come and go. Footballers aren't signed for their loyalty, moreso their ability to help the team win matches.
Expecting any more than that will only lead to a constant source of disappointment when players do in fact decide to leave.
How can a football club be anything or represent anything without the players. I mean sure we support the club and players come and go but in the time they are here their actions define part of the club's identity. In fact the players have to represent the club and it is that representation that can make you a fan of the club and get you to support it.
Playing football is their job, it is what they are paid to do but there is also a part emotional investment. Players must get a certain pride from representing the club from playing for it. You can't go through all the sacrifices players have to make to become good footballers if you only do it for the money. Sure they earn alot but money is not a very good motivator, that is science speaking. It only motivates to a certain extend and it has more importance of a demotivator, the lack of money demotivates players because they expect to earn a fair salary and they benchmark with other players. So beside a salary that a player earns there must be something that motivates them to play for a club else they could never reach the performances that are required on professional football at this stage.
That emotional investment is spurred on by the fans support. I mean go talk to a player and ask how it feels when 80k people are chanting your name when you are on the pitch. It is pure adrenaline, it is a kick. They want to play well and win matches and silverware for those fans in return for the support. But that support can't exist if the fans don't feel you are a part of the club, if you don't represent the club, are a part of it than such support is impossible. So yea fans pressume a certain degree of emotional investment from the players in the club else they wouldn't go out on a cold and rainy day to support them. Sure they don't support one individual but rather the team as a whole and in fact the club which has existed out of many different teams in the past but you are part of that team as a player, you are part of the club's history. So if the fans come out and cheer for the club, they cheer for you aswell.
The fans will always thank a player for great service and for loyalty. A player doesn't owe a club any loyalty imo but he does owe the club respect. The way in which you leave the club is an action to shapes the way you represent the club, it has to happen with respect for the club and its fans and beside a financial thing it is most certainly also an emotional thing. Fans need to understand why the player leaves and they need to be able to accept the way in which he leaves. The player must do everything he can to show that respect to the fans. I could accept De Geas reasons for leaving the club altough I think he leaves to soon and should have stayed atleast a couple more years also for his own career. But the way in which he leaves, with the entire contract situation feels like a dickmove from his agent. Just doesn't feel fair that we have to let him go in this situation, he should have signed a contract long ago.
I'd be far less disgusted with it if he had signed a contract renewal a year or two ago and would want to leave for Real Madrid now. I mean he'd ask to leave, we'd say ok if you want to than you can go but only if we can agree a deal with Real Madrid. And either we'd get fair compensation or Real Madrid would have to say no to the transfer.
Now it feels like De Gea (his agent) has forced us in a situation where we can basically not do anything but comply with his wish to go to Real Madrid and basically accept whatever it is that they would still want to pay for him. Feels like we have only become a detail in this entire situation, which feels wrong.