The “myth” has been observed. Just like Australopithocine, ignoring it doesn't make it go away.
What are the real world observations that would preclude the accumulation of a 100% difference in noncoding DNA given enough time? Functional constraints would, of course, limit the amount of change tolerated in genetic DNA.
You haven't even explained what would preclude the accumulation of the KNOWN amount of difference, some 2% genetic and 6% genomic, that has theoretically accumulated between humans and apes over some seven million years.
What would preclude two separate populations of the same species from accumulating enough differences in DNA over time that they could no longer reproduce between groups after sufficient time and the accompanying INEVITABLE change in DNA?
“when two different lines that formerly could reproduce fertile offspring can no longer do so”
This is not an observation, but a speculation. Speciation remains a myth.
“What would preclude two separate populations of the same species from accumulating enough differences in DNA over time that they could no longer reproduce between groups after sufficient time and the accompanying INEVITABLE change in DNA?”
Answer: the fact that genetic changes do not result in additional morphology.