It doesn't really matter what the encapsulation is/was, the values of a federated protocol the carrier participates in directly remain the same. The downside is you bundle the privacy to your carrier but that concern should really be solved with E2EE, not trust in a given provider. The upside is your communication service status is tied to your connection service status, and federated out immediately from there. You also gain the ability to fallback transparently to SMS/MMS in the exact same way RCS would work.
Google botched up RCS a bit in order to get it momentum, but plenty of carriers do support RCS natively as that's the only way Apple did it with iOS. Google did at least push E2EE options, but those only landed in GSM with RCS Universal Profile 3.0 and I don't think Apple has given a date for when they will support that profile on iOS. That is to say, the problems here are not inherent to RCS itself but the typical adoption and rollout problems of communication protocols.
All that aside, I'd gladly sacrifice the federated service provider flow if there were actually an equally popular federated solution to latch on to with full fallback capability to aid the remaining transition (+ the protocol actually be designed with radio power saving in mind). It's just RCS is by far the closest thing to that full package vs any other generic data messaging service.