I don't necessarily agree, but if we have a way to avoid undefined behaviour (and at least in C there are ways to make pretty thorough checks), then it works in practice.
If it doesn't work for C regarding mainstream adoption, how come it will work for Vulkan?
You will of course have the occasional cowboy (which I personally am, though in a different domain), but that shouldn't matter that much in the grand scheme of things.
Now if you ask me, Vulkan is not enough. What we really want is a stable, usable hardware interface. Basically an ISA. The thing will have close to zero bug, because hardware folks know how to properly test their designs. Undefined behaviour is likely unavoidable, but I believe it can be reduced to a reasonable minimum.
If AMD and NVidia started something like RISC-V, except for graphics cards, it will likely have a greater impact than RISC-V itself.
This is questionable. Vulkan by definition has basically no error checking in the driver, and while developers are supposed to use the validation layer, they may not do so, and even if they do, there are certain plenty of incorrect things an application can do that won't be caught by validation.
Incorrect programs may still happen to run correctly on existing drivers, but then fail with a driver update that happens to change the undefined behavior.