China has no need for outer Mongolia, even the most backwards provinces which are headaches for the government are not as backwards as outer Mongolia.Hopefully never, an independent Mongolia is a necessary reality check for ethnic separatists who believe they can make it out-there on their own. It's also helpful reminder to those who unironically believe chanting Western buzzwords will magically solve all their problems, and somehow westerners will make it rain if they chant them loud enough.
Just because the country can become larger doesn't mean it should become larger.
When Mongolia is its own country but dependent on China/Russia, Mongolia will send resources to China as well as what little useful talent they can occasionally make, all this is paid for by Mongolia itself. Whereas if Mongolia was reincorporated into China, the living standards, resource development and education of them must be paid from China's pockets.
Inner Mongolia is the part that is rather industrialized and has skilled workforce.