JSON was intended as a data serialisation format

Why then use a inefficient text based format instead of a much more efficient and easy to parse binary format?

Just use DER encoded ASN.1 like a normal person.

If no one trains the next generation of developers, who will maintain and build the systems of the future?

Senior developers like me, for a price.

They aren’t popular because there already is a very popular linux based mobile OS: Android. What would be the point of another one? Why would anyone want to use a new OS with zero app support and no advantage to using it?

If you want to overcome the obstacle of being a new platform with no support then you have to provide a significant advantage to make it worth the pain, and there simply isn’t one.

Why did people switch to iOS and Android phones when companies like Nokia had the market cornered? Because they offer a massive improvement in UX over the established players. What advantage do those OSes you mentioned have?

Android is Linux based and arguably the most popular mobile OS.