Is iMessage the default sms client? It's just called Messages on my phone. What does it offer? Mine looks like the stock android one. I see virtually zero options to do anything else than to send a message. Is it US only thing?
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.
Namely: Telegram, Signal, LINE, WhatsApp, Discord, among others.
That is, all except one.
The one exception is, of course, Apple's iMessage.