It's not wishful thinking, unless your power company charges you a constant rate in BTC for a kWh regardless of the exchange rate. At $400/BTC, once the difficulty updates in a few hours someone running an S1 at the default 2J/GH with 13c/kWh power will pull in ~$1.24 a day in BTC, and pay ~$1.25 in power costs.* At $800/BTC, you'd still be making $0.62 a day.
That's all good, but it does not invalidate the argument that in most of these scenarios you'd be much better off by buying BTC or holding the ones that you have. In other words if you need BTC to reach $600+ to break even on most current hardware, then why not just hold the BTC until it's $600 and decide then if you still want to buy the hardware, especially if it's priced in USD. I did just that earlier this year with some S2/SP10 purchases and they turned out great, much better than if I purchased the hardware a few weeks earlier at a worse exchange rate. If BTC doesn't reach $600 or whatever the target is then BTC is still in your wallet.