I don't think the letter goes far enough.
If Apple is doing something you don't like, put pressure on them. A better strategy than OP's would be to go to greater lengths, like labeling Apple "anti-developer" (the same way Trump uses labels against his opponents), but too many here are too scrupulous and rationally-minded to do this. The basic fact is, the tactics you need to use in a philosophical debate and the tactics you need to use to effect change are different, and, to be effective, you need to be a bit radical and a loose-cannon. Activists who insist on being factual, moderate or respectful, lose. That includes political activists, union leaders, as well as anybody that wants to change corporate behavior or policy as is the case here.
Unions are an old-school idea. But unions won't work to protect iOS developers from Apple.
So why doesn't someone build up a brand, blog, or platform as a "developer's advocate"? A Twitter account, speaking on behalf of 100k followers, has leverage influence, and power. They can represent people, organize communities discussing developers' issues, and speak out with a single voice. Why does nobody do this? It's easier for Apple to ignore 100k individuals than for them to ignore 1 organization with 100k followers. The latter will get much wider media coverage.
Social media empowered mobs, why not use that for good?
I'm honestly curious what makes it sound immature though. Could you elaborate?