Byku I'm in the US and barely anyone here actively uses WhatsApp, Signal, or really any third-party messenger. Everyone sticks to the default messenger and since iPhones are the most common, that means iMessage.
Even if they do have an active account or app installed from a third-party messenger, they don't use it and it's become a meme to be irritated whenever someone tries to message you on anything other than iMessage. The "Green bubble" dread is also an issue, not just among teens but among full grown adults that still act like children. Here's a pretty comprehensive video from a popular youtube tech reviewer on how messaging in the US got so weird if you're curious.
both parties using Google Messages app
Google managed to set Google Messages as the default messaging client on Android and even got Samsung to switch to using Google Messages instead of Samsung Messages. So, nowadays, if someone has an up to date android device, they almost definitely have Google Messages installed and are likely using it as their default messaging app already.
both parties having RCS enabled
Google made RCS turn on by default last year. Despite the difficulties of getting it to work on GrapheneOS, it's a frictionless process on stock.
both parties having access to network via mobie data or Wi-Fi
At least in the USA, that's a very low bar and it's basically unheard that someone forfeits a mobile plan, especially since most people here buy their phones through their mobile phone carrier under contract. Free wifi is also available almost everywhere. Data coverage maps have also improved a lot for the 3 major US wireless networks. People these days even have wifi on planes and utilize satellite internet services when in the outskirts or out at sea. So, this is pretty much a non-issue 99.99% of the time. Further, other messaging apps also require data or wifi so not sure why this is relevant...
While iMessage is still king in the US, Google Messages is still the second best alternative since I at least get E2EE with the few android users I know. But, now that Apple is rolling out RCS support with some hearsay that Apple will implement encryption in the future, that's even more reason to keep using Google Messages. Even without encryption between iPhone users, it's still enough to make some things break less such as sending videos and larger files, which makes dealing with tech illiterate apple users slightly less frustrating.
Obviously, these aren't the best messaging platforms for privacy. But that doesn't really matter when no one uses the alternatives. This is also a different use case. If I'm exchanging information with someone who is also knowledgeable about privacy, I may get them to temporarily install Signal, SimpleX, etc. But that's very infrequent. Most of the time, it's just day to day messaging with mostly tech illiterate/apathetic people who would rather tell me to drop $1,200 on a new iPhone then for them to install a third-party messenger...