I'm the same. Use other messaging apps for E2EE and other advanced features, fall back to SMS of there's no data connection or to ensure compatibility with the person on the other end.
The call network side is universal. Messaging apps aren't. It's really that simple for me.
With SMS, I don't have to agree with the other parties about what app we're going to use. We can just exchange messages without a hassle.
The only downside is the lack of encryption, but I can live with that.
Another consideration, and this is why I'm resistant to RCS, is that I actively don't want all the bells and whistles that messaging apps have. All I want to do is be able to exchange short plain text messages.
There are people who do not want to install four hundred messaging apps. There are even people who do not want to install one messaging app. Calls and texts are included in phone plans, and despite Facebook's marketing, there are still more SMS users than Whatsapp users.
There is no ideal cellular messaging protocol. People aren't going to change programs because some computer person is enamored with a specific encryption platform.
What's interesting is that in other countries such as Mexico, there is no such thing as SMS. Yes, SMS exists but nobody uses it. Nobody. You either send a WhatsApp message or you can't communicate.
It's the same situation in Japan. I've lived here for 5 years and literally never, not once, sent or received a message to anyone else here in Japan using the iOS Messages app. LINE messenger penetration is 100%. You don't ask for someone's phone number, you ask for their LINE username. And if you want to call them you use LINE's voice calling feature (which uses data so doesn't cost you minutes). I don't have anyone's personal phone number aside from my wife. Even my elderly mother-in-law makes calls using LINE.
My iOS Messages app is just pages and pages of phone number verification and 2FA texts, and then one group chat with remote coworkers abroad.
Most people use sub-$200 handsets, so the penetration of iPhones is very low and only the high class uses them. Within small social circles (mostly within the family), they use iMessage. But to communicate with anyone else, they use WhatsApp. I know a few high powered lawyers. They use WhatsApp exclusively.
WhatsApp penetration in Mexico must be about 99% in my estimation.
I really, really, really don't want SMS "upgraded" in this way. If I wanted something like RCS, I'd be using a messaging app.
Here's hoping that everything remains compatible with old-school SMS.