It's very simple actually.  The massive influx of hash power applied to lesser coins that will result from Eth going POS, will result in a massive increase in difficulty.  Higher difficulty = lower earnings.  These are simply inarguable facts.  Or to put it very simplistically, the same size pie will be cut into a thousand times more pieces so each hash will generate 1/1000 the earnings than before.

I find the blind optimism that the Eth POS will be a mere blip in mining profitability to be somewhat bewildering.  Only the strong will survive the aftermath.  "Strong" in this case meaning those with the latest gpus and/or the cheapest electricity.