Goals is goals.
Have you never learned to get better at something even though you did lots of it?
Like guitar maybe?
It's like i'm 21 years old but played guitar in a band for 5 years.
I can play an awesome solo to make Eddie Van Halen jealous but still can't muster a simple D major chord.
Sure, Lukaku might actually keep working on the poor parts of his game and improve them to a decent level. But the fact he hasn't managed to do so yet after 5 years of competetive football at a high level suggets there's a pretty good chance he never will.
As for the fact he scores plenty of goals, what would you (or anyone else) prefer : (for example)
Lukaku scores 20 goals next season but continues his poor hold up play, the rest of the team only score 20 between them and we finish 10th, or :
Lukaku scores 20 goals but improves his hold up play and helps the rest of the team score 40 goals and we finish 4th?
Thats why he overall play is an issue and me and others look beyond simply his goal tally, the better he does at other parts of the game as well as goalscoring the more we'll score as a team, thus get more points.
EDIT : and yes, the manager should also play to his strengths to help the team as well, it's far from all Lukaku's fault.