I don’t understand why the control panel UI wasn’t modernized instead? Would that really be unfeasible? I think it still might have been less work than to maintain 2 coexistent “settings/control panel” apps and migrate from one to another. Sometimes you have to throw out the old code base and start from scratch. But if you do so shouldn’t you rather distrubute the result when your finished and not in a half-baked compromise-like state?
RCS is an open standard, but Google’s implementation of it isn’t AFAIK. That’s why there exist no 3rd party RCS client outside of those praised by Google.