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I think this is the primary reason. Apple only announced working on RCS support very recently. Once that’s out, I don’t really see a place in the market for this.
And it isn’t just compressed images. MMS doesn’t support reactions, replies, typing indicators, or read receipts because it’s ancient. A proper, standardized replacement has been long overdue.
Granted, I’ve heard that RCS is currently heavily reliant on Google’s own servers, so it could be argued as to how “open” this really is.
RCS protocol is open. The apps… not so much.
I think the currently available apps not being free software is less important than the protocol being open, which is good. It allows for the possibility of FOSS clients in the future. My bigger concern at the moment is if most/all of the actual backend infrastructure is controlled by a single stakeholder.