Your point isn't necessarily reliability but availability.
It's both. If it's not always available, it's not reliable.
It's an interesting perspective to hear that the US appears to be so behind (at least from a European perspective of course) when it comes to messaging apps
I don't think we are, at all. Sure some other countries have a unified messaging platform, but all of them are corporate-owned, controlled, monitored, and not federated. In that regard it's barely a step up. Ideally we would have something like email (but of fucking course not email, because its atrocious) that doesn't require any corporation or centralization involved, where we could run our own or choose which entity we want to host our data, and would be interoperable across entities. Anything short of that I wouldn't consider much of an advancement (except Signal).
But again, that appears to be far more common in the US?
The only people that I talk to on Signal are the ones that I've been able to force to use it. If they send me messages on FB, IG, SMS or whatev, I just wont reply to them. I think there are quite a few more that use WA but I don't know because I refuse.
Do you ever miss the extra features that web messaging brings, like in-chat polls, voice messages, etc...?
Nice to have but not necessary and not worth it.
I'm not sure how much of that RCS supports (because almost nobody uses that here).
Much like iMessage, Google Messages is the messaging app that comes pre-installed on most Android phones, so people who have those phones use them. There aren't many but there are some. RCS is pretty dreadful, currently. iMessage is by far and away the most popular chat platform here, and is largely responsible for Apple's local dominance in the smartphone market. Especially among teens it is WILDLY popular. Apple is moving to add interoperability with RCS soon (thanks EU).
There are phone plans available here that don't offer SMS messaging anymore
I've never heard of such a thing.
I wouldn't be entirely surprised if SMS eventually just gets phased out.
Not around here, for sure.