I certainly wake up in cold sweats worrying about how we'll make up for Bony's 0 minutes last season.
Not to mention the vast majority of Navas' game time actually came at RB, so Kyle Walker & Danilo replaced those minutes and 13 games into the season, Bernardo Silva has played 1/3rd of Nolito's minutes in the front line already, so it's more than safe to say he has replaced those minutes.
As for Iheanacho, he played a gigantic 98 minutes last season when Gabriel Jesus was available. The rest of his game time came before Jesus arrived, so in answer to that question...Gabriel Jesus replaced him.
Gabriel Jesus has still spent more of his career playing LW than striker, and we are more than capable of resting Sané by playing Jesus-Aguero-Sterling, not to mention Bernardo Silva has himself said he'll play on the left as well as the right.
OK, so which mediocrity in the current squad is Sanchez going to replace?
Lastly, just because this is hilarious -
As I said from the beginning, if Pep can keep 5 world class players happy despite sitting on the bench for 40% of the season, than I will be ecstatic to see Sanchez come in, but it's never been done before in the history of the game, so questioning if it's possible is a valid concern.
Two things are definitely wrong in your post though.
1) He is not needed to replace Bony, Nolito, Navas or Kelechi's minutes
2) Guardiola
definitely views him as a striker, so Aguero is very much relevant.