Not sure about GP's specific case with Ethernet, but for some tasks that "little hardware logic on the side" can be the difference between some bit-banging operation taking one clock cycle vs. 200.

There are plenty of cases where the sequential logic for an FPGA may end up resulting in something looking like a custom CPU implementation. Development wise it may end up cheaper to just pick up an existing architecture and build in custom instructions for the specific parts that make it particularly expensive (either computationally or latency wise) for a traditional CPU.

How common are MCUs with mii or gmii interfaces? Certainly more mii than gmii, and at what price points? Not very common and not too cheap in my experience.

Because of supply chain issues, for one.

Is this chip somehow special and prone to supply chain issues? I am just curious.

Edit: I get tons of marketing materials daily about new various electric components. Fun thing is that while marketing department is sending advertising I can buy the parts mostly only in 2023.