I am not against signing a player to not get a resale value at the end. The resale value is, however, an important economic condition because it gives a club the chance to sell an asset for substantial funds and potentially replace that asset efficiently.
Moreover, the lack of resale value means City take on all the risk here, there is no room for error or that 125m+ is a total bust. It's the difference between the Griezmann/Hazard and Felix deals; Atletico have an out because other clubs would take the chance on Felix but no club would take the chance on Griezmann or Hazard for example.
The other key issue is the number of years of service you would get. Kane is 28, it isn't inconceivable that he could slow down on the wrong side of 30 or even earlier.
I posited this question before, how much would a 22 year old Kane be worth? Of course it would be a hypothetical situation, but if a 28 year old Kane is 125m, a 22 year old Kane would be 250/275m+?