this is a terrible comparison, there's exactly 1 place for a CDM in every lineup, there's 2 places for CB's and 2 places for number 8's.
In comparison, we have 1 place for a gk, and guess what, we're not buying a 100m backup goalie either!
We absolutely have back-ups - fernandinho is a backup. we don't need to spend so much money on a player who would be second fiddle, this is greedy.
I also reject arguments in the vain of "well what happens if X player gets injured" - well what happens if Ederson gets injured? you play steffen. what happens if rodri gets injured? You play the very capable CDM that you've gotten as a backup. 30m. 35m. That is a completely reasonable amount of money to spend on a backup, not more.