I don't think you can get away with converting it to a spectrum and simply saying "well they are all being rotated to some extent". Let me convert something else to a spectrum for fun. All the employees in a company are not equally important, but all are important to some extent, so let's use that to justify giving £300k salary to the receptionist. That's a spectrum of usefulness as opposed to a spectrum participation time.
The players that are more essential play more games and are more important than the others. Pogba, Lukaku, De Gea, Valencia, Herrera, Mkhitaryan are those players. When they are fit and the games are important, they'll play those games. Such would not be the case for Matic, since his entire position is being rotated in and out of existence. Thus spending £50m on a nearly 29yo that will only occasionally be used seems madness. At least spend it on someone we'll get more years out of. He's also not versatile to play any other position. Only the one we don't use in half our games.
I really can't fathom how of all the DMs out there we settled on Matic and are willing to pay £50m for him, more than what Chelsea paid for his 22yo replacement. It's not a position with short supply of players. Or one that is pivotal for us to function.
