1) They had a problem with Islamic terrorism associated with Xinjiang and see this as a solution

2) Eliminating Turkic, Mongolic, and steppe peoples has been Beijing's and Nanjing's policy since like 1700 (with a kind of interlude 1949-2009) https://twitter.com/ClaireBerlinski/status/1361604484615577603

There are basically two theories of what China should be which have vied for power in the courts of China's rulers for centuries:

Should China be the Han-dominated areas of the east, profitable and valuable on their own right but without secure borders?

Or should China take on deeply costly and troublesome money-losing territory inhabited by locals who dislike being controlled by outsiders, but in exchange have buffer zones around the Han core, and access to various extractive resource benefits?

Inner Mongolia, Xinjiang, Tibet, even Manchuria are all part of this debate. Today Manchuria is so integral it's difficult to image a China without it, but the other three are all a pretty straightforward counterfactual.

A China without Inner Mongolia, Xinjiang, or Tibet is a China which is considerably richer and easier to govern internally, especially if Qinghai and western Gansu go in the process.

But it's also a China which faces an *extremely* severe security dilemma.

The experience of the "Century of Humiliation" as well as Manchu domination mean that China's current rulers feel that security dilemma VERY acutely. The outlying regions are security investments.

It will be interesting to see if this changes over time. As the Han settlement project in Xinjiang appears to be flailing as Han people flee the inconvenience of the security state there, will it get costlier to hold Xinjiang?

As it does, and as conventional wars become less common while insurgencies become more potent, will China calculate that holding Xinjiang is a bigger security threat than leaving it be?

Probably not anytime in the next several generations.

But it's not hard to *imagine* a world where we get to 2050 without China getting embroiled in a war, it's in terminal demographic decline even as it is relatively prosperous, the security dilemma fades from memory, and the rapidly-growing ethnic minority populations get restive.

In this scenario, the obvious path is that the "Autonomous Regions" actually get real autonomy. Xinjiang and Inner Mongolia become like Scotland. In an extreme scenario, one can imagine a Greenlandic type relationship.

Not because these regions where wrested from China's control (that's impossible; it would be a bloody and inhumane nightmare for a foreign power to try to wrest Tibet or Xinjiang from Beijing's hands) but because eventually China just decides it isn't worth the trouble.

Independence is a lot harder to imagine. Surrendering territory is politically difficult for any state, especially China.