We can interpret the move as 11.0 is the first ARM OS Version… They cannot go back to 1… macOS 1 ;).
Right. I’d not be confident with “MacOS 1” 
But unless they planned, since the beginning, to never change anything (CPU, core software components, brand or anything else) for the next 100 years (at least), I can’t see how the claim that X (10) would no longer change was made, if ARM is already enough for that change.
Way off topic, but that wasn’t the case when they moved from PPC to x86 …
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I had that in mind when writing that.