I do agree it's not always profitable for everyone, but here we talk about hypothetical connection between number of miners and profitability, and the fact it will always be profitable for someone, so we are going to have miners and blockchain will continue to run.
Difficulty raises, but as well reversed proportion between number of miners and the shares they got. Under assumption farms are closed in whole world, it will become profitable for mining with CPU again :)
That's the 'small math paradox' they forgot in "Anti BTC" campaign.