It's not a "clear alternative" when Apple doesn't even allow you to mention that. There are other reasonable alternatives such as letting developers mention the alternative payment methods as long as they explain what the benefits of Apple payment ecosystem.

I read a lot of people writing comments like "I prefer to go through Apple because of the ease of unsubscription." If the benefits are so clear, why are developers not even allowed to mention the alternative?

Android is a very real and clear alternative. Android dominates market share. How you can say there isn't a clear alternative, I don't know.

Given the vendor lock-in and Apple's 50% market share, it's not much of an alternative.

That doesn't justify your argument. 50% is not a dominant position.