Maybe, but it's a problem either way. Mongolia now has a border with just two countries: China and russia. Their merchants can use whichever has lower tariffs or better security etc. So there's a small bit of leverage, which gives a bit of freedom. Now, if China gets the part of russia that connects Mongolia with russia as part of a peace deal that splits russia among the allied forces like Germany after WW2, Mongolia will share a border with only China and they can put tariffs etc as high as they want. If China closed their airspace they couldn't even export/import by air. It would be very worrisome and an immense pressure.