Is that interventionist policy any different to the Fed and ECB propping up of the US and European stock markets since the depths of the crash with QE? If not then when is the long term viability of that solution going to be called into play?
The Fed and ECB's goal is not to prop up stock market valuations, but to help the economy by encouraging spending and investment through moderate inflation. The banks are tightly bound by inflation targets. Arguably, the ECB has done too little QE since nominal GDP has barely (or hasn't) recovered in most of Europe since 2007, and inflation has been way under target since the crisis.
It seems like China's reaction is more panic-driven, and doesn't have a framework such as inflation targeting to constrain it. In addition, this intervention is just to prop up stock asset prices; encouraging spending and growth in the whole economy is not the primary goal.
With respect to "China's reaction is more panic-driven", check out the following three charts of US Fed policy which I would argue were "panic-driven" based upon magnitude of the intervention when compared historically.
It remains to be seen to what affect recent US Fed policy will do long term, however, I would argue that it was definitely a panic driven reaction.
I provide this not in support of China's policy but for context.
I think the better statement would be that it was a panic, but one with very different and more practical goals.
The US government was primarily trying to keep some big companies and banks that were caught in a liquidity crunch from going bankrupt, because them all going under would likely be a spiraling problem. The crisis solution was also rather simple in aggregate...loan them a bunch of money temporarily until they could free up the assets to repay it.
China is currently trying to prop up a stock market that is STILL (even with today's decline), up 50% in a year for no reason. And most trading in the market is done by very jittery small investors (who are also overleveraged and now desperate to get out) rather than big firms and funds. Said small investors are unlikely to be reassured by anything the Chinese government can possibly do. I don't see great chances for the government to be able to stop this from returning to a more realistic valuation.
Not to mention that I can't imagine how illegalizing trading and other knee-jerk reactions could possibly instill confidence in the market. Even if they can slow the decline, it seems like they'd just enlongate the decline until it reached a similar reality (if their actions don't actually increase the sell-off out of fear).
