That's not necessarily true. For full compatibility you'd need a pre-installed app on Android, but vendors like Samsung and Sony and OnePlus can build their own RCS messengers for those devices should they choose to. Same with custom ROM developers for an open source implementation.
For non-ROM developers, it depends on what RCS activation technique your carrier uses.
RCS isn't a Google spec, or an Apple spec, or even an IETF/IEEE/ISO spec. It's part of the core mobile networking specifications. It was created by the people who designed the MMS spec after 4G switched mobile networks to everything-over-IP. Unfortunately, the 4G spec didn't require RCS, it was just an optional side feature, so carriers never bothered with it.
RCS solves the problem that most of the US uses iMessage or SMS/MMS, but the SMS/MMS part of that equation is absolutely dreadful. File size limits are stuck in the mid 2000s, messages are split over multiple SMS packets if you send more than one sentence, the entire thing is unencrypted. Sometimes people like to send photos to each other and the 150KiB or so file size limit on MMS isn't enough for that anymore.
As for why not have people install Signal: why would they, because everyone is already using something else? I live in a country where everyone uses chat apps and all but one of my contacts are on WhatsApp. In other places, that'll be Telegram, and in some North American countries, that'll be iMessage/MMS, the texting app that comes with the phone for sending texts.
I don't believe this FUD. I think overal the market has moved from from charging for messages. I fully believe carriers would love to rip off everyone as much as they can, but I think carriers know customers would just use facebook messenger if they tried charging per-message.
Carriers charging too much is exactly why WhatsApp became so popular in so many countries. It was optimised for very little data usage, so was dramatically cheaper than SMS.
Don't forget you could also send photos with it, which is more than what can be said about MMS.
Funny enough, I think I was charged once for a MMS in the past 5 years. No idea how I managed to do it, but it seems to still exist and still be charged per message even on my unlimited everything else plan.
For non-ROM developers, it depends on what RCS activation technique your carrier uses.
RCS isn't a Google spec, or an Apple spec, or even an IETF/IEEE/ISO spec. It's part of the core mobile networking specifications. It was created by the people who designed the MMS spec after 4G switched mobile networks to everything-over-IP. Unfortunately, the 4G spec didn't require RCS, it was just an optional side feature, so carriers never bothered with it.
RCS solves the problem that most of the US uses iMessage or SMS/MMS, but the SMS/MMS part of that equation is absolutely dreadful. File size limits are stuck in the mid 2000s, messages are split over multiple SMS packets if you send more than one sentence, the entire thing is unencrypted. Sometimes people like to send photos to each other and the 150KiB or so file size limit on MMS isn't enough for that anymore.
As for why not have people install Signal: why would they, because everyone is already using something else? I live in a country where everyone uses chat apps and all but one of my contacts are on WhatsApp. In other places, that'll be Telegram, and in some North American countries, that'll be iMessage/MMS, the texting app that comes with the phone for sending texts.