I highly doubt Khedira is intended to be a bench player for us, 4m is not the salary you give for bench players in Serie A, Sami would be the top earner in all Serie A clubs bar Milan and Inter where he'd be 3rd and Napoli and Roma where he'd be 2nd highest earner.
we might be more well off than rest of the league, but we're not stupid rich, I don't see how we're going to be able to add another high earner like Pogba or SMS to our collection of Khedira (just renewed), Matuidi (rumored to be on the brink of renewal), Pjanič (just renewed) and Can (just arrived), obviously our current midfield is here to stay.