> AFAIK the microcode updates aren't mandatory, you can use your computer without them and use the stock microcode (though that's also proprietary).

Microcode is only one part of it. I was thinking of the ME firmware, and to a lesser extent the FSP. It's not possible to boot a modern Intel processor without ME. The ME has direct DMA access to all peripherals and can use the network interfaces directly, behind the operating systems back.

I believe AMD has similar things. They are all signed by the manufacturer and the hardware will refuse to load a replacement even if it existed.