IMO, people make an error by equating our lack of scoring with selling Lukaku. It's a bit more complicated than that, it's the fact the we don't create enough, don't have patterns of play upfront, don't have enough creative midfielders who can spot a pass, don't have an adequate #10, don't have a RW, don't have a LB etc. You can get prime Ronaldo on this team and he will suck, let alone Lukaku who needs excellent service to score.
Lukaku wanted to go, he can go. It's a mistake to keep a sulking player, who's being a distraction, a man child, a pain in the arse, unprofessional by tweeting out internal physical data etc. He had to go, and at that price it was the right decision.
The club knew Lukaku wanted out since April, which was enough time to scout and find a replacement. They didn't get a replacement, for whatever reason. That's the mistake.