Does China still want the North Korean buffer zone?

Yes and here is why.

First, we need to look at some recent history of China and maybe we can get some idea of its reasoning. 

In the 1800's the British East India Company began trading goods with China but China would only accept payment in silver. the britishs were getting nervous about the amount of silver leaving their shores and were looking for something to trade that would bring some silver back. that product was opium.

The problem with this was that too many Chineses were getting addicted to it and not coming to work so the Emporer suddenly banned it. 
The britishs did not like this and so the opium wars began. 

China were defeated and forced to open more trading ports, then the French got in on the act, then the Americans, Dutch, etc. 

It's a good article and you can read it here

http://nationalinterest.org/blog/the-buzz/the-opium-wars-the-bloody-conflicts-destroyed-imperial-china-17212

After that,  Japanese wanting to expand their influence in the region, attacked in the Korean peninsula in 1894. at that time, Korea was a client state to China but Japan saw Korea as a convenient source of iron ore and coal and a tug of war began.

Again China lost and had to cede Korea to Japan.
And if that wasn't enough, they invaded again in 1937 occupying a good part of China until the end of World War 2 in 1945. 

So maybe we shouldn't be surprised that China intervened when UN forces got too close for comfort in the 1950 Korean war.

So you see, China has had many years of invasions and defeats and now realizes that it has been vulnerable to attack from the Pacific coast on numerous occasions.
These events have left a deep scar in the Chinese mindset and they realize now that when things go bad in Korea, it's bad for China as well.

fast forward to today

Korea is divided into North and South with the North supposedly having close ties with China. But it's more of an alliance of convenience really. the two countries do not like each other.

The uneasy alliance has its advantages though.

First and formost for China, North Korea provides a good buffer zone for stability in the region. China values this above all else as it has its hands full watching its other borders.
the last thing it wants is an open border with United States forces. 

China has easy access to North Korean coal and minerals and North Korea gets the hard cash it needs.
North korea gets its oil and food from China so a big leverage over North Korea there.
China obliges by sending back North Korean defectors and the regime likes that because it limits the amount of information that gets out to the world about how bad North Korea is.

Underneath the surface though, relations are not as close as is publicly portrayed.

It has even gotten to the point where there is speculation of debates in China about weighing the benefits of keeping the regime propped up or letting it fall.

Here is an interesting interview with Ely Ratner of the council on foreign relations (CFR) relating to this. 

https://www.vox.com/world/2017/8/10/16125076/china-north-korea-donald-trump-xi-jinping-kim-jong-un

There was some hope that Kim Jong Un might change the status quo but this has not been the case.
From his actions, he has shown himself to be incompetent, arrogant and inconsiderate of his people. 
Building an amusment park, day care centers and multi million dollar ski resort only available to the relatively wealthy while his people struggle? this still blows me away...how irresponsible can you get.

He is resistant to Chinese style economic reform and has become more rigid in his thinking. Punishment for "illegal" cell phone use has become more severe. the methods of tracking calls to China have gotten more efficient also.
This indicates that North Korean society is becoming more aware and hungry for outside information and the regime is afraid. it is afraid of the population becoming aware and seeing the regime for what it really is. a more aware population and a rigid government will eventually clash and this is probably has China nervous as well.

http://www.dailynk.com/english/read.php?num=14745&cataId=nk00100


There is evidence that the gulags are expanding indicating that more prisoners are being added. not that China cares about this but I'm just giving the reader an idea of Kim Jong Un.

http://www.telegraph.co.uk/news/worldnews/asia/northkorea/10496464/North-Korean-gulags-expanding-satellite-images-show.html

The execution of Jang Song-Thaek was a blow to China NK relations as he was the main intermediary between the two countries. no doubt this had a profound negative impact on chinas attitude toward Kim Jong Un. 
It is rumored that Kim got wind of a conspiracy to replace him and had Jang executed. 
Maybe it's true, Jang probably saw the new leader for what he was and realized he is going to be bad for North Korea.

http://www.asiapress.org/rimjin-gang/report/arrest-displacement-suicide-purge-jangs_affiliated_cadres-continues/

And finally, China is feeling pressure from around the world regarding North Korea nuclear development.every time NK launches a missile, the world looks to China to do something and this puts pressure on China and it makes them look bad.

I can understand why NK is doing this though. they see it is a deterrent to invasion but it could turn into something else down the road.
North Korea has seen what happened to Iraq and Libya when they gave up their weapons of mass destruction. NATO just walked in an took what they wanted. these invasions and occupations were based on lies by the way. Iraq was taken by direct force and Libya by proxy.
NATO has been trying to topple the Syrian government (again based on lies) and would have by now if it weren't for Russia.

So the regime looks at this does not trust the western powers.
It knows its conventional military would not withstand a NATO assault and so the nuclear deterrent is the regimes key to survival.
South Korean and united states joint military exercises every year simulating a war with the north does not help matters either. 

I just want to add here that the corporate media coverage of North Korean nuclear capability is overblown. I'm talking about the delivery systems and quick deployment capabilities part of it. they are at this point, not a threat to mainland united states.
The western corporate media are just parrots for governments and corporations and are drumming up the usual fear and tension but that is for another post.

So in summary

The North Korean regime is a pain for China and it is not happy with Kim Jong Un.
It would like North Korea to allow economic reform Chinese style but that would mean opening up to the rest of the world. The NK regime is so afraid of being exposed for what it is that it can't let that happen. Its crimes are too grat.
Kimg Jong Un and his inner circle would be hanging from lamp posts in no time.


If it is in China's best interest to let the regime fall however, it will let it fall.
It doesn't want that though because it means that it would have to deal with a flood of refugees and would have to intervene somehow in the Korean Peninsula.
China also knows that the united states is declining economically and in time will not have the influence it once did so maybe China is just waiting it out.

Having said all that, here are some good reasons why China might just let the regime fall.

http://www.scmp.com/week-asia/opinion/article/2075170/north-korea-loses-its-use-china-loses-its-patience