I'd wager the number of people using Messages on macOS is really not that high. Most people are content to message using their phone.

iMessage absolutely is a lock-in for iOS, though.

Why's iMessage a lock-in? I have an iPhone but I wasn't aware I'd lose anything (significant) by switching to Android (or whatever).

The biggest thing I lost in a similar switch was ANY old conversation with somebody who was an iMessage user. Where they reply to a conversation and think they are just texting you, but actually they are sending iMessages that you just aren’t receiving. Especially on, for example, family group texts, people just find the old conversation and continue it from the previous year or whatever. Lots of times when I was on windows phone or had iMessage switched off, I’d only get the parts of the conversation coming from non-iMessage users. A good example is random family members who use android texting me reactions to an original text or picture I didn’t get. It’s really dumb. Maybe there’s something I could have done about it in my own settings, but instead I ended up switching back to iPhone recently. Not just for that reason but it sure was nice to get messages properly again. But also more sensitive stuff like somebody sharing pictures of items related to planning a funeral.

It's not well publicised but, if you know anyone who's moved away from iOS they can remove their phone number from iMessage here;

https://selfsolve.apple.com/deregister-imessage/

Thanks, that is really interesting. I hadn’t read that page, but I had disabled iMessage from the phone settings, which is how I first noticed the issue. Maybe I was not thorough enough, (it says to disable FaceTime too, which I didn’t/don’t want to disable) because it never corrected itself.

I have also heard that if you complain and drill down enough in the support funnel on this issue that eventually the best answer will be "change your phone number". Now that's what you call lock in.