Can't really said that he is not stable because of the fact, Lukaku's performance even more stable than Osimhen, which is far younger than him. He played on the different teams in two seasons, but he can still adapt in a short time. This makes me think that if Lukaku is even better than Osimhen. Though he is still quite old., but it's not a problem at all.
Just because you have a young talent, and it doesn't also support your opinion if he is better. Osimhen has more time to develop himself than Lukaku, but i can't really believe if Osimhen can do what lukaku to play in different team every year.
I don't believe Lukaku is a better player than Osimhen currently. Osimhen gives you more than just goals, he plays with the team. He joins in the build up of the attack, he's also a hard worker. Lukaku was part of the reason Belgium got eliminated in the Euros. He waste too any balls. Any ball that gets to his feet gets wasted. I agree he knows how to score goals, but if he's not in a goal scoring position he can't do anything with the ball. We saw how instrumental Osimhen was for Nigeria in their afcon, I can't say the same for Lukaku in the Euros. Osimhen may not be an elite striker that can beat players and score, but he can score and he's currently better than Lukaku.
I believe Napoli have already told Conte they plan if selling Osimhen that's why he doesn't count on him. If Lukaku was that good Chelsea would have kept him after buying him for 100 million.
I believe the reason Napoli is finding it hard to sell Osimhen is because of the price tag they put on him. Nobody would buy Osimhen for 150 million. They can sell for about 80-90 million max. Chelsea are interested in a direct swap deal but Napoli wants more more. The same thing that made PSG pull out of the sea. I think it's looking like Osimhen will remain at Napoli.