There are much better arguments that brown/polar/black bears should be considered a single species, and that they perhaps would be if they were first discovered today. It is true that the distinction between "two closely related species that diverged recently" and "two distantly related varieties of the same species that are diverging more and more" is quite vague and subjective - especially when there are large phenotypic differences.
However, few of these arguments apply to human populations. With the exception of dogs, cats and other domesticated animals, few or no other mammal populations that have diverged as recently as humans are categorized as separate species (for example, brown and polar bears have diverged ~600k years ago, before our species even existed). Human populations only began spreading out of Africa some 150k years ago, which is not a lot of time in evolutionary terms at all, for a mammal that only reproduces around 1/year and needs more than a decade between generations.
Not to mention, the separation between all human populations is merely geographical. When different human populations came into contact, even ones that had been isolated from each other for tens of thousands of years, they virtually always interbreeded in significant numbers, contrary to, say, wolf and dog populations. This is also a strong argument against considering distant human cousins different species.
Additionally, morphological variation among isolated human populations is actually quite small, compared to intra-population variation. Skin color is the single biggest morphological difference, by far; followed by the size/hardness of our hairs, especially the ones on the head. Apart from these, virtually all other differences are more about statistical prevalence of certain traits (e.g. blue eyes are more or less common in various populations, body hairyness is more common in some populations than others, tallness is more common, etc).