First, it is really disheartening to read Leoforos Syngrou's tale of dedication, inspiration, and going all-in with iOS development only to have his app rejected. I hope Syngrou can find the wherewithal to continue on his journey of software development and I think one place he might actually try is with Apple itself.[0] He's already worked as an iOS developer and I also think he might try working for another iOS team (or as a contractor) in order to fund another foray into independent iOS development.
Or maybe leave the Apple ecosystem and take his considerable talents elsewhere. Basically, I'm saddened to hear he's burned out and wish him as speedy a recovery as possible.
Finally, and this is going to go against the grain of the hacker ethic prevalent among many of us HNers, I think it has to be said that being anticompetitive may be unethical but it is not illegal in the framework of US antitrust law unless the company uses a monopoly position to extend its dominance in other markets.
Independent developers and second-party shops must be very cautious and circumspect about their business prospects if they do not have a formal contract with the owner of a platform for which they are developing.
Syngrou should be proud of his accomplishments and not take Apple's rejection of Windmill as the final say about his skill and promise as an iOS developer, let alone a developer of his caliber and dedication.
[0] I understand this is a difficult and uncertain road, but it's worth a shot. I say this as someone who works on an Apple campus.
EDIT: fun -> fund; "among most of us HNers" -> "among many of us HNers"