The issue is with 80 million they cant buy an 80 million striker. Like leicester if they sold mahrez for 40 there isnt a ready made 40 mil winger leicester could buy.
They can buy someone cheaper and hope he turns into an 80m striker but thats what he would have to do to replace lukaku.
Same sort of thing as suarez at liverpool for a while and bale with spurs. You get the money which goes on lots of players none of which replace what youve lost.