If the mining reward plus transaction fees doesn't exceed the cost of operating an ASIC, I am quite certain those boxes will be unplugged.
To assume anything else is to claim that the majority of miners are irrational.
In such a scenario, is it more rational to unplug your ASICs and keep them in your basement, or to sell them to any takers and try to recoup your capital outlay?
In the latter case, OP's argument prevails, because the market value of an unprofitable device would be low, and the people who would buy it in such a situation are ones who would mine no matter what the exchange rate might be - whether because they have ideological ("irrational") reasons for doing so, or because they have an insignificant marginal cost to mine (e.g. people who don't need to pay for their electrons by the kilowatt-hour).
The point is that if you have hardware with no purpose other than mining, the market will eventually put it into the hands of a person who's willing to use it for that purpose - or, in other words, that in the long run all ASIC miners are online and mining.
