The argument seems to be two fold. either;
We is going to be world class, he is only young and will get better and is proven in this league.
And;
Despite the fact he is only 20 he has a poor first touch and only scores more than any other Everton Striker for 20 years each season and i'd rather buy someone else who is probably not for sale.
Its simple, Lukaku came in and was our highest scorer since Andre Kanchelskis nearly 20 years ago (apart from the yak). He is only 20! He is at the beginning of his career and will not hit his peak for another 7 years at least. He has proven to regulary score in this league and people still question why we should sign him? He isnt going to turn into Messi overnight but it doesn't mean he isn't good.
I mean, i have used this analogy before but it fits.
Romelu Lukaku has scored over 30 goals in 2 years as a teenager in the Premier League and he does that for the next 10 years he will be scoring as many as top goal scorers do in the same time. Drogba, who is a very similar player to him, didnt start playing top flight football until he was 24! So Lukaku has 5 years on him already.