'Mongoloid' people migrated into Central Asia from further east, and mixed with the native 'Caucasoid' people. Most Central Asians have essentially become a mixed-race people, and so in all the Stans, as well as in Xinjiang, you will see both 'Mongoloid'-looking people and 'Caucasoid'-looking people.

Tajiks are very much Caucasian. In fact, all Central Asian ethnic groups are at least in part Caucasian. Peoples like Kazakhs and Kyrgyz are mostly Mongoloid, but they also have a share of Caucasian blood. Central Asia has always been a crossroads where races and ethnic groups met and mixed.

As for the Tocharians, they were assimilated by the Turkic Uighurs. Today some of their former centers, like Turpan, are still inhabited by people who are of part Tocharian blood. Who by the way look very interesting.