After four different CPU ISA's (68000, powerpc, x86, ARM) an Apple ISA is next.
6502 (Apple 1,2,3): 1976
68k (Mac): 1984 (+8 years)
PPC: 1994 (+10 years)
x86: 2006 (+12 years)
ARM: 2020 (+14 years)
If the trend continues, we will see Apple's next architecture in 2036.
That being said, I would really like to see the Mill make it to Silicon - even if it doesn't work in the end, it's much more interesting to me than RISC-V.
I assume RISC-V appeal is because it's open and thus allows people to explore (proprietary) implementations while maintaining compatibility at the ISA level.
If I understand correctly you cannot do this with Mill either.
Their work looks very interesting, but I'm not drawn in writing an fpga implementation or work on a toolchain for it, since I know any success may be impeded by patents
Meaning they can do anything they want on an ARM base, they're already not beholden to any third-party designer or roadmap, which is what hampered them with both PPC and x86 (I'm not old enough to remember the 68k and really know why they moved off of it).
The only reason they'd have to move away from ARM is if the ISA ends up preventing them from doing something, somehow.
The actual CPU ISA will be a closely guarded trade secret. It will, however, run Swift code 2-3x faster (on a single-core basis) than any commercially available commodity CPU -- especially since Apple pays silicon fabs to not manufacture 3nm hypercubic gallium-arsenide ICs or whatever the cutting edge is on behalf of competitors.
In fact, it wouldn't surprise me if future versions of Xcode did all their compilation in Apple's cloud, without an option for local compilation.