How many kids from any Premier League Club ever make it to the top ?.
Each and every Premier League club must have thousands of kids come through their doors from age eight and onwards, yet virtually none of them make it, bar the odd exception - Rashford, Rooney, Barkley, etc. The majority ending up being let go or ending up playing in Lower leagues.
What is the point of have an academy, when it's cheaper and easier to buy a ready made foreign player who can come straight into the team ?.
Academies are necessary. I think what you describe is more a consequence of the quality of the academies and the natural talent that is present. This sort of determines the quality of the footballer that you deliver. Also the frequency of delivering a good one. They are also a possible way to develop a house style.
For premier league clubs almost all the players don't end up playing for the first team but I don't see this as an issue. They can still form a source of income. It's not a realistic expectation to expect a high number of successful graduates in times of globalization.
Some academies also prepare the less successful ones for a 'normal' life. For instance: The Purple hearts academy sends them to a special school and such.
I also think academies are necessary anyway because they are a way to integrate the club into the community, even though lots of the players aren't from the area. A youth policy is very important for a club. Specially for smaller clubs since they are able to get revenue from it and supporters (all the parents go to club organized activities and such) - not for premier league clubs obviously. It also makes it a somewhat achievable goal for young lads to go and play for the club of their choosing (youth levels are less strict; still difficult though).
I enjoy watching academy teams.
For instance:
Anderlecht academy: delivered: Romelu Lukaku, Tielemans (really like watching him), Kompany, Januzaj (hopefully he can turn it around)...
Standard: Witsel, Carcella ...
Genk: Courtois, Casteels, De Bruyne
Lille: Junior Malanda, Hazard ...
Man United: Rashford, Lingard (to some extent),
...
I've only named somewhat recent graduates.