Doesn’t everyone hate it when google extends APIs? Think it’s called EEE (embrace, extend, extinguish). They have a history of killing standards as soon as they have enough market share.
If you change off of googles servers you lose features. I’d consider that no longer an open platform. So despite not needing to use their implementation, if you want the modern features RCS is often advertised as having, you have to go through google. That’s not an open standard.