This is actually a common occurrence. How often do we hear players dubbed the 'next Messi' or 'next Ronaldo' and then become nothing. Goncalo Guedes at Benfica was one such example. He went to PSG because of this moniker then ended up on loan at Valencia because he wasn't good enough. And before the Neymar/Mbappe comments are mentioned, it shouldn't have made a difference if he was as good as was hyped up to be.
The 'Hazards' situation at Chelsea is one more example. Yes Eden is fantastic. But his brothers; Thorgan, Kylian (and a third) are/were given such monikers and all went through Chelsea at one stage or another and only the third is expected to come good these days. Thorgan got shipped off because he's not good enough. Kylian isn't far off either.
But as has been said, clubs don't produce wave after wave of first teamers from their academy. You're lucky if you get one a season. Most teams buy younger players from other academies and then develop them further into first team players.