it is completely on Apple.
Every carrier has a "carrier bundle" which explains how the carrier works with the phone. For non-Apple phones these are edited, managed, and released by the carrier themselfs. For Apple phones they are baked into the phone by Apple, and Apple only allows you to have a custom profile if you sell enough apple phones.
If you do not have a custom profile, you inherit the "generic" profile of your parent provider, so GoogleFi uses the "generic" MVNO profile for T-Mobile.
These profiles have two new possible keys in them with apple iOS 18.
`EnableRCSByDefault`
`ShowRCSSwitch`
Both of these default to False. As almost all cariers world wide use google provided backend for RCS known as jibe Apple could set ShowRCSSwitch to `true` by default, and default to the generic Jibe backend when a user switches it on.
Apple could also work with their existing profiles to update them to have this value to true for any provider who offers Jibe to their MVNOs.
Apple could also allow carriers to edit/update their profiles to allow smaller carriers such as Google Fi to have a profile beyond the generic T-Mobile profile.
This is absolutely, and without a doubt, Apples problem, Apples fault, and Apple's job to fix.