"Wage structure" is just a way to say that we have to consider what the other players in the team earn. Sure, Salah compares himself to what Mbappe earns, but players like VVD and Trent also look at Salah's wages. We can give Salah the highest wages, but the difference to other top players can't be too big. Otherwise they will get rightly angry and look for a pay increase as well, and we cant afford to pay 10 players 400 k/week.
Regarding ManU wages, they should be used examples for caution, not as a point of reference. They pay insane wages for average players, and that is precisely why they are so bad. They would pay 200 k/w for someone at the level of Ox, but that doesn't mean that we should do it.