Most mobile operators in the UK have stopped with SMS limits (unless you’re on the really really cheap plans or PAYG). Guess people just don’t use them enough to warrant caps on it.

I can see why Google doesn’t allow swapping out specific “core” apps such as settings. Giving a foreign app access to your system settings sounds ripe for abuse, which is why a lot of permissions are focused on changing them.

RCS does function over a data connection (and WiFi!), however unless you’re sending large files over the wire it’s probably not going to have any effect on your data bill. Text messages are a handful of kilobytes large at worst. SMS/MMS have lots of issues to do with security and capability, and most handsets support it in some form already.