Comment by numpad0
RGMII is not some obscure competitor to Ethernet, but rather, Ethernet was designed to be a modular two-part design with "MAC" and "PHY" chips connected via "MII" interface. RGMII is simply the latest version of it.
Many Ethernet-supported SoCs still use various MII style interfaces because it makes more sense to outsource the physical layer to some external chip especially if not everyone is going to use Ethernet.
It's perhaps like the difference between using Thunderbolt vs raw PCIe. You technically shouldn't need Thunderbolt if you're just permanently connecting two things inside a same machine.
Is it smarter to do it proper and make it silicon efficient than just shipping the darn thing ASAP? idk. We'll see.
RGMII requires way more work to run board-to-board (heaps of signals, quite precise length matching, impedance control, etc. on the boards, better board-to-board connectors etc.) and at the end of all that will likely be less robust than just running Ethernet. I'd much rather use SGMII just because it's far fewer signals to match (even if it runs way faster) instead of RGMII.
The chips they're using might already have Ethernet PHYs built in anyway which might also be part of the reason they're using Ethernet.