And in a business, if you talk about selling now, you need to look at current value. Pogba's current value isn't even half of Marchisio's.
Essentially, what you said is exactly how you should not run a business. Even if I don't agree with the notion that football it a business, just like any other.
I realize that Pogba, at present, is probably worth 20 million Euros, but we aren't going to sell him, so it doesn't really matter. Value is not just about what you or I think, but what the market dictates, how much the club value him, and how much other clubs are willing to fork out for a specific player. Hence, I am totally against selling Pogba, because in a couple of years he will be worth a lot more, and because I highly regard him as a player.
As for Marchisio, an offer of 35 million should definitely be taken into account. That is my opinion. You are free to disagree with it. I am sure there are others who wouldn't sell him, even for 50.