For that matter, even the “isolation leads to speciation” theory fails...I pointed out examples where that is not correct.
Your counter-arguments (i.e., "there is a grand difference between lack of desire from that of lack of capability") do not change the fact that interbreeding does not occur at the endpoints of ring species.
In these cases, isolation has indeed led to speciation. No theory needed, just observation.
Isolation does *not* lead to speciation. If it did, then pigeons in New York would be different from pigeons in Australia.
And “ring species” are no such thing...just the same species in different colors.