Look at the situation.
The Russian leadership are revelling in the situation where a rogue nation is now able to genuinely threaten it's main opponent. Why would Russia want that to change?
Same for China. Without the US interference in the SCS who would there be to oppose China's monopoly over resources there?
There is simply no will or motivation to act. I don't believe either nation gives a damn to be quite honest whether or not the peninsula is unified on South Korean terms. Both super powers are content to continue the charade of pretending they are acting but in reality they are laughing their pants off.
If DPRK were pointing missiles at Russia or China, then Kim's nation would be ashes long ago.
It's the end for the DPRK and the US knows it. Already Ambassador Haley has hinted that the UN has run it's course and she is happy now to hand the matter over to the Pentagon.
U.S. Ambassador Haley: U.N. has exhausted options on North Korea | Reuters
I reckon this is it. There is no more road to kick the can down. The US is correct in that. Kim is not interested and sees that he is holding the aces.
All we can do now is pray that the opening USAF attack is totally effective and that Seoul is spared.
Short of the US employing a naval blockade (and a land blockade) to prevent oil and major supplies getting through which in itself would most likely trigger an all out conflict then the USAF has to go in and perform a massive first strike.
I predict that we will wake up one morning soon to the news that hundreds or thousands of sorties have taken out the DPRK's missile and artillery batteries and the rest is just a matter of time before the DPRK's military machine is rendered ineffective.