The point is these areas are still pretty actively Tibetan today, not just historically.

Is there a contiguous region stretching from Mongolia deep into the Middle East and to Europe where a high percentage of the population (like a quarter or more) speaks Mongolian as their native language?

I know that might be true for a small area surrounding Mongolia in Russia, but I think the comparison stretching into the Middle East and Europe is not accurate.

Qinghai is still almost a quarter Tibetan (a percentage that has probably been consistently shrinking in recent history due to migration) and is almost as large, area-wise, as the formal Tibetan autonomous region.

The dedicated Turanist will talk your ear off about linguistic co-evolution, haplogroups, and so on, but no, of course not. The example was deliberately outlandish. A quarter is lower than the proportion of Russians in Crimea.

I think we may ultimately be making the same point, that political borders are by nature artificial and often fail to reflect the historical fluidity of culture and ethnicity, but I objected to the perceived implication that these regions are uniquely Tibetan.

Right, "It simply isn't feasible to grant every cultural group the full extent of their historical claims."

Doing this will extinguish multiple continents...