Crux of the debate right there. Has he plateaued, or is there more to come?
People go on about his goals, but a few things - first of all, he scored a lot of them in Belgium, hardly a massive standard of football overall. And secondly, for West Brom, Belgium and his explosive start at Everton, he has scored the vast majority of those goals in a counter attacking system of football (he went 15 games for Chelsea without scoring in a possession based system, and has largely been uninspiring when we've played a similar system too).
These are the doubts. People are not getting on his back for "fun", there's a genuine element of doubt that Lukaku is the top player a lot of people think he is or he is going to be, especially when employed in a system that to all appearances seems not to suit him at all, and he's made no improvement in for several years now.