What I'm saying is you never know what's going to happen on a single heavily controlled platform. In 2 years time, Apple can create a competitor to your app and ban you (happened), create new guidelines so you can't publish your app anymore (happened), change search algorithms and your app drops (happened?), remove you for political disagreements (happened), remove your app because of API deprecations and non-backward compatibility (happened).
Developing on a platform like this in inherently risky, this is probably fine for (most) companies but I would advice developers to avoid devoting all their careers to a single platform and try to diversify a bit in case something happens in the future.
What I recommend doing is to learn how to develop solid, cross platform APIs and you then have the flexibility to develop as many clients as you like, web, iOS, Android etc. with no single point of failure.
It's not good but it's workable. Which Apple accepts as a reason to focus very few resources on dev tools.
Yes, this letter should probably reference a bug#, but it should not be in the bug itself. And, because it's an open letter, you can go add a bug and make a PR to add it to the letter.
Those are the important things, quibbling about some features in the development system is silly, esp. when Apple has poured tons of resources into Swift and done it really well.
Messenger has had CallKit integration for quite a while now.