The issue is these contracts don't work in isolation, if you give rashford 500k a week because it's cost effective, your wage structure now incorporates 500k a week and the next signing decides thats what they want. And the one after, and your money you saved not replacing rashford is now spent in the inflates contracts of other players
It also totally ignores the concept of negotiation. It might cost us more to replace rashford, but he might place a value in staying in England, staying in the Premier league.
With de gea off his contract, varane hopefully doesn't get renewed at 350k a week, ideally we'd shft sancho, we'd want to move back towards a wage structure. Again look at Liverpool, they've been a far better team than us, with far better players and pay lower wages than we do, because they're well run and have a wage structure. And offering 20m a year contracts to any player who threatens to leave on a free is not how a well run club operates. Look at Madrid, flo does not bow to player power and as a result they're the best run club in Europe, their rivals who let player power run amok are probably the worst run club in Europe.