To be honest at the time he left I probably would have agree with you but now he's left and I've had time to think on it I don't agree at all.
Lukaku is a player that absolutely loves to be wanted by someone - it's why he left Chelsea to come to us despite them being his dream move and it's why he then ended up leaving us for United and then United to Inter. Each stage of his career he's been wanted a bit more by someone or by someone he deems as a bit better which is why he's moved on. In the case of United he didn't have the manager he wanted to play for anymore in Mourinho as Ole took over and things were turning very sour at United, with the club and the fans. He knew that Conte wanted him so it was an easy decision for him to leave in the end.
SAF was excellent because even if you weren't his favourite he was able to make you feel like you were his favourite. Every manager clearly has them but the best ones are capable of keeping squad harmony and making each player feel special and want to go that extra limb.
Unfortunately also for Lukaku he decided to 'bulk' for the WC in 2018 which took away his absolute best attributes which were his power, pace and drive from his lower body because he had this upper mass compared to his leaner stronger frame. It completely changed his style of play from the one he was at Everton and United (first season) and in the second season it just wasn't reversible quick enough for him to turn it around. Once Conte came in for him, you were given the chance to recoup the fee for him and Ole figured out the style he wanted to play with United it was always going to be curtains.
It's turned out to be the best move for all parties. You've got Martial who suits you much better under Ole and Lukaku is back to his old leaner self and enjoying his football again. The mistake was not purchasing an adequete replacement for Lukaku in the form of a player who was good enough to challenge Martial and keep him on his toes/replace him in games like last night when you needed a goal desperately and he just wasn't connecting properly.