You are comparing apples to oranges. People are not going to run large scale mining needed to secure today's network at a loss. You can't compare that to a few people running CPU's or GPU's. Mining profitability is a major issue for Bitcoin survival.
I totally agree that no one is expected to act unless there is a profit motive. But that is how it scales. When prices are high it is worth the investment in equipment. When they crash hard and mining is not profitable, some mines will close. Because of open competition we should always be at about the lowest profitable transaction fee. That may not be enough for a large scale mine, but it will be enough for someone. The question is whether it will be enough to secure the whole network to a sufficient level that people will use it. Very difficult to predict how this will scale.
