Miners don't provide a service by selling. That is a dis-service.  It's also not important since there will be still sellers without miners.  The less sellers, the better.  Price in free markets is always set by supply and demand.  Less sellers (less supply) will eventually take the price higher unless demand is non-existent.  We don't want sellers, that's the opposite of what is needed.   If there weren't so many people dumping into the bids, and instead, refusing to sell, the price would rise.

Then when the miners all jump in one pool/group or superior hardware such as fpga's concentrates the hash power, then we see unintended or malicious forks.  As these forks keep happening, investors lose faith in the dev's ability to keep the network stable and teh price dumps even more.

None of this would be an issue with POS.