Milik plays in a different spot in Ajax than in Poland. The role he fills in Poland is played by Davy Klaassen in Ajax. I see Milik more as a real nine, though he does play the odd match in Ajax as shadow striker as well but only when Klaassen is injured. I am still on the fence regarding Milik, he did play a very good second part of the season in Ajax but in the Polish team he was back to wasting chances which he had been doing the first 2 years in Ajax as well.
There is also still the chance Milik leaves Ajax which would leave us with our recent Colombian purchase (Mateo Casierra) as only true nine. Ajax is also still interested in getting Klaas-Jan Huntelaar as extra attacker, while old he would nevertheless still wreck face in the Dutch league where defending is more of an afterthought.
EDIT: However, not signing Vincent Jansen in the winter break was simply stupid because Milik's departure is imminent no matter how you look at it, and Ajax does not really have any option besides Milik there either. Zivkovic is a major flop, and let's be honest Anwar El Ghazi is not a nine unlike Ajax sometimes seems to think he is.