The reasoning is quite sound, yes, but it doesn't consider the long term implications. No one outside of the USA is using iMessage and they are one regulation away from losing it in the USA too. Also, outside of the USA if they piss off Mark Zuckerberg enough they can lose the rest of the world because who would want a phone that doesn't have WhatsApp?

It also demonstrates the harm with these lock-in strategies very well. They are concerned that their product wouldn't be compelling enough for people to buy it.

Wouldn't be better if they owned the messaging platform and provide the full experience for iOS due to the fact that they control the device? Owning the platform would have given them also the ability to get revenue from non-Apple users through things like payments and other services that can be integrated into the app.

In its current situation, it might be the case that Apple has won the battle but might end up losing the war.