So if I would escalate this to absurdity, then you would say that if currently City would offer 50 mil for him, we should take it anyway, right? Because for the reasons you mentioned, this season is the best opportunity to sell him and in the future he will be worth less...
Value of an asset is always combination of it's intrinsic value and demand. This season we can see that for whatever reason, there is not enough demand for one of the best players in the game.
But situations change. Many things could cause a ripple-effect what comes to top striker demand -
- PSG might loose Mbappe and they need a new superstar without counting money
- Real Madrid might not get Mbappe nor Haaland, have weak results and want a new Galactico
- ManU might just nearly miss out on some trophy, loose Pogba and want to take step forward and sign Kane
- City does not seem to get a striker in and might once again just nearly loose out in CL and next year can come back for Kane...
I mean there are lot of possibilities. It is not as linear nor simplistic that his value will surely decrease.