That’s configurable. By default a user can access them from other devices, but that feature can be disabled.
Your turning it off has no effect. Most messages are backed up twice.
It doesn't matter how much better it is, if the only way to use it is via Apple hardware. It's their prerogative to do that, and it makes good business sense assuming it doesn't bring them monopoly problems (and IMO it probably shouldn't due to the aforementioned stats re: worldwide usage at 27%, and even US usage is only at 55%).
But it in no way is a standard. It's just a proprietary piece of software. So literally any other standard proposal is "more of a standard" than iMessage.
I want to against this. Even if the app is well made, messaging platform only for specific company's devices is completely broken idea. Tragedy is that US people adopted completely broken platform.
I text Android users all the time and besides the green bubble I don’t see much difference honestly
Apple built something better. So did Google. Then Google killed theirs. Then they did the same thing three more times.
It's no wonder nobody wants to use Google messaging, and that Google resorts to smear campaigns. People would rather use WhatsApp spyware from FB/Meta or totally unencrypted Discord/Slack than bother with another Google IM product (which, let's be clear, is 100% what RCS is).
PS: Apple preserves a backdoor in the end to end crypto of iMessage for government surveillance that allows Apple to read all iMessages. iMessage isn't good either.