I’m curious of the history. Presumably people used SMS when it was the only option, and stopped at some point and installed message apps (but they never stopped in the US).
I think this is all stupid. One the one hand the tech companies get slammed for not inter-operating, then Apple gets slammed for inter-operating. Loads of phones have come with vendor specific messaging apps and such, it's just that Apple committed the crime of making theirs decent and successful.
Come to think of it, the same lack of interop openness applies to airdrop, use of the on-phone NFC chipset (for uses other than apple-pay), and a bunch of other such things.