Aguero would be sold because there's no way of having Mbappé, Sanchez, Sterling, Sané, Jesus *and* Aguero.
It's a really obvious idea, Mbappé is going to be a once in a generation player so you move Aguero on a year or 2 early. You miss out on a few goals this season but are paid back several times over in time and you've also got better interplay between the front 3 so you get more goals overall even if Mbappé scores fewer individually than Aguero would.
I don't want it to happen, I want to see Aguero stay and I'm pretty sure he will, but I completely understand the reasoning behind the move if it happens.
If it's Sanchez and Mbappe then i could understand yes, but that wasn't wat the OP said he just said Mbappe would put doubt on Aguero's future to which I'd disagree and I'm sure you would aswell.