I understand and agree with you to a certain extent, but we're not just talking about a couple of assembly commands that could be misused. The Intel ME is a FULL Operating System running MINIX Linux (edit: MINIX is not Linux, as corrected by @dragonwriter). It has it's own network and apps, that run inside a running kernel, of which you have no access to.

Even if the intentions are 100% legit, this is an operating system that you can not update (as frequently as your main operating system), and has many attack vectors.

Yes, it could pretend to run your firmware, but secretly load it's own, but it's actually quite hard to hide a 5mb (2mb min) piece of firmware in the chip. Research microchip decapping. You can clearly see the different regions of the chip.

But yes, it could be possible to hide a few x64 instructions, or circuits that could be manipulated. But running a remote control environment that can share your screen without your knowledge can only really be done clearly by running a large separate application stack alongside your main chip. (For now, who knows where we'll be in 5 to 10 years).