Messages sends SMS/MMS (on a Phone or configured Mac/iPad/Watch), but will transparently upgrade to the iMessage protocol when talking to another Apple user. This has substantially better features over SMS, including network access and the ability to send higher quality multimedia.
If you don't ever want to fall back to SMS, you pick some other app (WhatsApp, Signal, etc).
In the US, unlimited texting became a thing much earlier than in the EU, partly because the carrier and network relationship is structured differently. So SMS is bad but free, and thus a bit more tolerable.
A higher percentage of iPhone users means that more often than not, you'll find your text is using the much better protocol. As a result, many in the US never had to pick a third party to be "winner" via network effects (like say LINE in certain asian countries).
This puts things into a weird state, where SMS and iMessage sort of act like a single pseudo-"product" in the US available to both iPhone and Android users, but where Android users get a way worse experience and where iPhone users get a worse experience when talking with Android users.
Which is where I get to personal opinion, and say this is mostly Google's fault. https://www.theverge.com/2021/6/21/22538240/google-chat-allo... . As owner of the other major platform and with the ability to release a compatible chat app for iPhone, they've had and squandered every opportunity to own the space.
Your summary is right but is missing one key detail. In the period since iMessage was developed, every single direct messaging and group messaging app that got to scale has been x-platform across Android and iOS, and many of them have even been x-platform to web/browser clients and/or desktop apps across Windows, macOS, and Linux.
Namely: Telegram, Signal, LINE, WhatsApp, Discord, among others.
That is, all except one.
The one exception is, of course, Apple's iMessage.
Thanks for the details and background. If I recall correctly, back when I had an Android, every app (Signal, Telegram, Messwnger I think) wanted to become my default messaging app. I'm assuming to do basically do what you are describing. Google had like 20 messaging apps over the years, but they never combined ot or I guess other vendors went with stock sms-only one instead?
Android has a concept of a default SMS client, but not one of a default messaging app that isn't an SMS client. Signal, for example used to include an SMS client but no longer does.
If you don't ever want to fall back to SMS, you pick some other app (WhatsApp, Signal, etc).
In the US, unlimited texting became a thing much earlier than in the EU, partly because the carrier and network relationship is structured differently. So SMS is bad but free, and thus a bit more tolerable.
A higher percentage of iPhone users means that more often than not, you'll find your text is using the much better protocol. As a result, many in the US never had to pick a third party to be "winner" via network effects (like say LINE in certain asian countries).
This puts things into a weird state, where SMS and iMessage sort of act like a single pseudo-"product" in the US available to both iPhone and Android users, but where Android users get a way worse experience and where iPhone users get a worse experience when talking with Android users.
Which is where I get to personal opinion, and say this is mostly Google's fault. https://www.theverge.com/2021/6/21/22538240/google-chat-allo... . As owner of the other major platform and with the ability to release a compatible chat app for iPhone, they've had and squandered every opportunity to own the space.