Some inside rumours about miners planning on moving to mine ETC when ETH moves to Proof of Stake. But regardless, I don't see gpu sales declining soon as there will almost always be crypto to mine, be it for speculative of current value, ie. immediate profit.
With regards to Goldman Sachs, I think they're just throwing their marketing weight around for their own personal gain. I don't believe they came out with that statement to be helpful to the community or share valid information with use small fishies. No, it was and always will be for their own personal gain. But notice how smaller and smaller an impact any FUD is having...