Biologists have *two* mutually exclusive definitions of speciations: any change or the inability to cross-breed.
As for inability, there is a grand difference between lack of desire from that of lack of capability.
That the same salamander “can’t” breed with the same salamander but of a different color is preposterous. That one color salamander wouldn’t *want* to breed with the same species in a different color is far more plausible.
Some white guys don’t fancy black girls. No big deal. But they *could* breed with black girls. Big difference between “don’t want to” from that of “can’t do it physically.”
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.