There's no point comparing to other sports, wages have to be structured around the game we have. According to Spotrac Chelsea's highest earner is Lukaku on £16.9m a year. If the club suddenly give Rudiger £20m a year then the clubs wage budget is going to explode over the next 12 months as all the other players suddenly demand massive raises.
You can certainly make the argument that football is headed that way anyway, and in a few years it will be fairly normal, but from a business perspective it would be madness to jump-start that process before you really have to. If we lose Rudiger and bring in say Kounde, we don't become a radically weaker team, and we certainly wouldn't need to pay him £20m a year.