Isn't it obvious that China could make North Korea a high-income country faster than Russia could???
In principle, yes, but:
-DPRK doesn't produce anything to achieve it normally; Korea has stuff China needs, but nothing at this scale.
-Temporarily, DPRK has that much stuff Russia needs.
-China isn't really interested in big, prosperous North Korea: it's quite unruly when it is poor and dependant. Imagine Northern voice when it'll grow arrogant.
-neither DPRK really wants to be more dependant on China than it already is. North Korea is a country saved by China, and they know it, but so do they know rest of their history.
Good analogy is that China is DPRK's pension, and should remain this way. Russia is our Kim winning a lottery. Great, comes without obligations, but it's temporary.