Well, what of it? Mongolia managed to secure independence because both Russia and China were in total disarry during the 1910s and the regimes nominally in power found it easier to acknowledge an independent Mongol buffer state than use up more energy trying to (re)take the region. But what if, say, the early Republican period in China didn't go so terribly wrong so early on? Or, what if Mongolia is 'recovered' by the Nationalists in the 1920s or '30s? Likewise, what if things go a bit badly during World War Two and the Japanese go to war with the USSR early, making gains in the far east and toppling the Soviet-backed Mongolian government before eventually cocking things up themselves? Could the CCP have gotten Outer Mongolia after the war through some contrivance?
Specifying ROC & PRC as a way of avoiding Qing-continuation possibilities, which would make this question more complicated than it already is.
Specifying ROC & PRC as a way of avoiding Qing-continuation possibilities, which would make this question more complicated than it already is.