This aspect is ignored, because it's clear that Apple blocks third-party clients to maintain its dominant position in the US (social unacceptability of green bubbles among teens).
If cost was the problem, they could offer a subscription.
It's pretty clear why they don't want an android iMessage app.
In this case, what beeper enables (if successful) potentially is to use Apple's infra for future communication between android to android phones, or android to iMessage groups, while on Apple's infra and dime. Beeper will likely collect a fee for it as well. Thats not a position Apple would want to be in.
So I'm imagining Signal, Telegram and WhatsApp on my iPhone? And the appeals to emotion really have got to stop.
People do not by iPhones because of iMessage. I'll totally accept that some, even a majority, buy them as a fashion item, in a similar way that Samsung S series phones are, but iMessage will not be a significant driver for many.
If cost was the problem, they could offer a subscription.