For one it would be really nice to be able to have a computer without a highly insecure and backdoorable management engine inside its CPU, basically all x86 computers sold since 2010.
How would you know that other than the assurances of the company that designed the chip? They could easily insert a few gates in the layout to subvert protections under the right conditions.
Same could be said for MIPS or RISC-V or indeed any piece of silicon.
As an embedded designer I know for a fact that CPUs used in embedded systems get so much scrutiny on their I/O messages all the way up the stack, it would be real good subversion to hide something like this and not be discovered in all use case scenarios. More likely is some sort of back door when you can access the hardware. Disclaimer I only work on the smallest MIPS and ARM embedded chips not those with full MMU's
With intel/AMD you have an assurance that your chip does contain a backdoor. I know this is probably difficult to do, but if we have access to published architecture specs/layouts, it might be easier to audit the chip design, even if doing so requires an x-ray microscope.
This is the crux of the matter. While your statements are perfectly correct, their implications aren't. The assumption with open source is always that it's easily auditable and anyone can see the source (code, floorplan, etc.) which somehow implies the finished binary or chip that you have received is "safe". And herein lies the problem:
1) Assuming the source is really clean there's no guarantee the end product is;
2) Even if the source appears to be clean you have no guarantee it was actually (thoroughly) checked because "someone else" always checks.
The best example I can give you is OpenSSL, a library used by most of the internet and hundreds of billion+ $ companies (and hundreds of thousands of million+ $ ones). It took 2 years for anyone to notice it. And there are far more qualified SW engineers around that could have spotted the bug than there are HW engineers capable of finding the equivalent backdoor in a complex piece of silicon. So I am very skeptical that someone would notice one if one was there.
> ...but if we have access to published architecture specs/layouts, it might be easier to audit the chip design...
there are no "architecture specs/layouts" provided by RISC-V. it's an ISA. companies making RISC-V chips can put whatever they want in them, and don't have to document any of it.
When you know you have a backdoor you may take some steps, small as they may be, to protect yourself. For example in some cases you can remove the ME blob from the BIOS, effectively disabling the ME. You can choose to drop the product entirely. You definitely can't protect yourself from something you don't know exists.
Considering nation state actors have been consistently backdooring everything they can get their hands on (network equipment, CPUs, software, storage devices, you name it - and they've done it without the manufacturer ever knowing sometimes) the assumption that such an open source project would not have it and it's "a lot better" is based purely on wishful thinking.