Icardi will not be a failure. He has 10 goals in first season in the most defensively disciplined league in the world, and he's only 20.
I've seen a lot of people that have watered down his goals, like oh 4 against pescara, 2 against juve, 1 in the moon, one was a header, one with eyes closed.
You can water down any striker's goals like that. Even players like Cavani/Falcao have so many penalties, that Icardi didn't have.
Those were 10 goals from open play, and were almost all amazing.
Again, remember this was his FIRST season.
If you look at a random striker's trends, with Icardi there is an almost 0% chance of a flop. I know everything has a risk, but this move is as good as it gets.
You can lose form and all, but his movement, dribbling, technique and eye for finishing you can't lose.
For a 20 year old to come and boss like that on his first season with a second hand team, it takes great mentality, that will be very hard to break down. With a coach like Mazzari that gets the best out of strikers, and in a system where he will have way more service than Sampdoria, the risk is extremely low.