It seems like people have a very basic misunderstanding of what SMS and RCS are.
SMS is carrier based messaging. It is the first form of messaging available on cell phones, from before there even were phones capable of downloading apps and accessing the interent. And so it is universal. It will work between any phone and any carrier. Someone on an iPhone can text someone on a dumbphone and it will work. There is no other universal system of messaging between cell phones.
RCS is intended to be the replacement for SMS, but to provide more modern messaging features. Nonetheless RCS is also carrier based messaging. But the rollout has been slow and messy. RCS is not dependent on Google Messages or iMessage or any other app. Google Messages in not another messaging app, like Signal or whatever. It is just Google's stock app for SMS and now RCS. For the moment, many Android phones (but not all) can only get RCS working through Google Messages. But in theory someday RCS will be just as universal as SMS. Although to be honest I don't know how or if RCS will work with dumbphones. If SMS is someday deprecated, perhaps older dumbphones will become obsolete and be unable to message in any form.
So SMS and RCS really are not the same thing as Signal, etc. Messaging apps are privated closed ecosystems with no interoperability and that require internet access. Some messaging apps, like iMessage, are platform and even hardware dependent.
It is true that in the U.S. generally people get unlimited SMS/RCS messaging with even the cheapest cell phone plans, so there is not the same incentive to switch to some other messaging app.
On the other hand, people elsewhere in the world did not adopt Whatsapp because it has e2ee. It was adopted to get around the cost of texting, which is ofen not free in many parts of the world. Whatsapp was very widely used and extemely popular long before it had e2ee and it was only recently that e2ee was turned on by default in all messages in Whatsapp. If people view Whatsapp as a serious privacy app, they are mistaken.
All that aside, I have plenty of friends in Europe who use plain old SMS for messaging. Yes, other messaging apps are more popular, but they are hardly universal. And in the U.S. probably half of users are on iMessage, which handles SMS, but between iPhone users is its own messaging platform, with e2ee, etc. So it is extremely false to say that all Americans just use SMS.
So I think for a lot of Americans, when they travel abroad, and people say, oh you can contact me on Whatsapp, you can contact me on Telegram, they feel like, why can't you just text? Because Americans are used to having a universal solution, that doesn't cost anything, that works between any phone, and doesn't require installing a new app, giving it access to your contacts, agreeing to terms of service, learning how to use it, etc. SMS may have many limitations (which RCS is supposed to correct eventually), but it's universality and lacking the requirement to install yet another app is extremely convenient. When you're used to that, Whatsapp, etc., seems like cumbersome cluttering up on one's phone with multiple different apps that all do the same thing, but only work between some people and not others.