The reason all modern CPUs looks the same is decades of evolution. We have settled on a way to do things that work, and guaranteed to work from a first try. That's how engineering works.
Not quite.
Evolution, both biological/engineering, makes
local optimisations which are based on the
history of environmental/technical conditions. It is effectively a "hill climbing algorithm". That leads to messes like the human eye, which a 10 year old can easily see is ridiculous (especially compared with some of the other eyes that have evolved).
It is frequently the case that adjacent "hills" have higher peaks and, if the chasm can be leaped, there are better rewards (a.k.a. a more optimal solution) to be found. That's well known in problems attacked by "simulated annealing" algorithms, e.g. much CAD software.
That is particularly valid when conditions change. In the engineering sense, Moores law is running headlong into thermodynamics problems. Yes transistors can continue to shrink for a few years, but they are very leaky (= hot) and are becoming small enough that there are fewer electrons in them than desirable.
If we want progress to continue, we have to try radically different solutions. The Mill is leaping a chasm in a particular direction.
People have tried many other architectures (VLIW, OISC, pure stack-based machines, register-less machines), and there is plenty of information on all sorts of strange stuff. But it does not get wide adoption as none of them actually bring performance improvement significant enough to justify re-doing all the software work that has been going on for the same decades.
Those strange architectures also often face problems interfacing with existing IP. How hard would it be to attach existing GPU to that CPU? Will it just work, or something about new bus structures will make it impossible/impractical?
There is a lot of stuff to think about, and the actual architecture matters very little on a full system scale.
True, with reservations.
The architectures you mention typically were pretty small deltas on previous architectures, attacked a small of proportion of the current challenges, and punted difficult problems into the future (assuming Moores Law would come to the rescue). That has advantages and disadvantages.
The Mill is a more radical departure tackling many of the existing challenges. It is reasonable that it should take longer to investigate and document. They are tackling many of the software issues head on, rather than using more transistors to conceal the problems.
So far I haven't seen a flaw in the Mill's architecture. I believe that it can be made to work, and producing initial hardware will not be a major engineering challenge (unlike making a faster x86!). Time will tell whether it succeeds in crossing the chasm.