I believe the other reason he appears to be very important for the team is that he has no competition in his position - I initially thought Boniface was going to be that guy to give Osimhen a tough challenge for the 9 spot, but he ended up being the biggest flop, hooked and dumped out of the National team.
Until we have real competition in that position, we will be overly reliant on Osimhen, and he has shown on multiple occasions that we cannot cope without him. He improved the squad when he is on the pitch.
Of course, the Super Eagle is nothing without Osimhen because he has the quality. One thing that I love about him is that he has he passion and zeal to play for his country. You will see him always motivating the players on the field as if he's the captain. I believe that we might not see anyone to compete that position with him because nobody has that confidence and passion of scoring goals.