The cream of world football in their age group is overstating it a bit when you're talking about a 9 year old (imo). By the time they're 15, yes, but by then, many of those kids will have been through more than one academy, so it's a bit misleading to think City 'produced' them - it's a tricky subject though - determining who has contributed most to the development of a player.
The other thing you're missing is that we are talking about the players who aren't rejected... by definition, the ones STILL in the academy are getting to play, but many leave the academy too. We're talking about a child at 9 - what are his ODDS of making it at the highest level? - 1/50,000 maybe. What are the odds of him making it at any professional level? 1/1000 maybe. IF a player goes to City does it increase his odds of making it at any level? or would sticking with Oldham be a better bet?
There's not much evidence to suggest that making City's academy for a year or two, then being dropped by us is any more beneficial - regardless of which other players you played with, or against. Plenty of lads who are 15 and spent most of their time at a lower league academy end up at a PL one anyway, and then go on to make it - because identifying talent at 15 is more reliable than at 9... (but it's still highly speculative).