Osimhen spent his first 18 months in Europe at Wolfsburg, where he played a combined ~300 minutes without recording a single goal or assist. He then loaned out to Belgium and only when he was 21 did he have his first good season in a big league, Mikel spent his first two years in Europe at FK Lyn in Norway. Iheanacho cost City 1 million, that's hardly a big commitment by their standards - the same summer they spent four times as much for 16 year old Angelino. Two of those examples actually show the opposite, if anything and even if they didn't three players over close to 20 years doesn't speak for much.
And if modern footballers are "mechanical" as you put it, it's because modern coaches want their players to be that way.
Speaking in general it's just common sense to me that the more focused attention and guidance you give to a child the easier it will find it to develop its skills and I don't see how millions pumped into youth football would only yield minimal advantages. An example to highlight the disparity would be Otto Addo, in the African system he's Ghana's head coach, in Europe he's a "talent coach", whose only job it is to mentor a few select talents at Dortmund. A while ago I read that Bradley Fink, by no means in the top tier category of talents, uses a personal trainer in his spare time to improve on his weaknesses in 1on1 sessions, on top of all the work he does at the club. You think all that doesn't have a clear effect? Even within Europe we can clearly see significant differences in the effectiveness of youth development.
I wouldn't say that African footballers are flat out worse, but it seems obvious to me that their privileged European counter parts have a significant head start over them and the African kids have to play catch up for a few years even after they move to Europe. Hence even all-time greats such as Mane and Salah having to move up the ladder step by step until their mid 20s, before catching their big break, hence someone like Osimhen flopping at Wolfsburg.
