@mjg59 I'm thinking more about things like SMM (and ME, and even UEFI).
What does SMM do that the EC couldn't do better?
All of the hardware I design these days has a MCU that does management stuff (sometimes two, one really low level one for power/reset sequencing and one that comes up later to do everything else) and then talks to an FPGA that does most of the work of the board.
The FPGA doesn't care about polling sensors or controlling fan speeds or anything else that's needed to make the system work. It just lives off in its own temperature-controlled world and does its thing, and has a SPI interface to the MCU when it needs something.