The problem is that SMS is nowadays a poor fallback, lacking support for a number of widespread features that are part of the EU's required baseline for interoperability.
The DMA requires interoperability without a decrease in security - this explicitly includes end-to-end encryption, which iMessage supports. Since SMS has no standard for key distribution/discovery, it already fails at that hurdle.
It also requires that features such as group messaging and audio/video calling.
So, let me get this straight -- the EU is now requiring end to end encryption? Haven't they been harping this whole time on how that enables child predators?