Outer Mongolia was disposable, no? The independence of Mongolia is because the PRC negotiated with the Soviet Union in return for Soviet assistance, although Mongolia was self-governing.
I'd argue that Lhasa, as a small city in a sparsely populated frontier region, is disposable. So's the SCS islands, but in a different way. The SCS is a Chinese strategic frontier that's being heavily fortified. Having the SCS get bombed out is not a preferred outcome, but it's preferred to having Hainan bombed out, which is the entire point of the SCS.