If Sanchez signs and PSG bid for Aguero then you obviously think it's a straightforward decision, we keep Aguero. You're saying that presumably because you think Aguero would play regularly. For me, I don't rate him as highly as Sanchez, jesus and Sane. I'd be tempted to sell him and bring in a top class defender if I could. It's about optimally managing the playing squad and players ability and form changes over the years.
The year after we won the title under pellegrini, Toure was untouchable and no one wanted to sell him, but in hindsight had someone come along with a decent bid, then it would have been best if we had accepted him. No player is bigger than the club.
I accept the argument that Aguero is too good at the moment to sell. That's one of interpretation and judgement. I don't accept the argument that because of what a player has done in the past, he should always remain integral to the team, no matter what.