I don't get this idea that Zaza can't improve and become better, considering he's 23 and quite a ways off from 24 still.
Look at someone like Diego Costa. 8 league goals at 21, 6 at 22, 10 at 23, and 8 at 24. Could anyone have predicted he would have become one of the most prolific strikers in the world at age 25 and 26? I doubt it.
Zlatan didn't really become a world beater until 26 years old. His first 3 years in Italy, he scored 16, 10, 15 in all competitions. At 26 years old he took off.
Zaza is hard working, he's hungry to improve, and seems to have a decent eye for goal. We're not going to sign some 30mm+ striker, so I'd be willing to give someone like him a chance, see what he can do. It's likely he never amounts to more than a decent-good rotation option. But what starting 23 year old strikers can you get for 15 million today?