Ah, but then things get more complex. If you are the primary wage earner, you can't accept a wage lower than what you require to keep yourself afloat, that's basic arithmetic. But...if your expenses are subsidized then you might accept a lower-than-usual wage. Some examples could include food stamps, public housing assistance, etc. In effect, a third party (the taxpayer) subsidizes artificially LOWER wages due to those safety-net benefits because your personal minimum you require is now lower. From an ultra-simplistic view, supply is meeting demand with low-skilled labor at that price, but is that "fair"? Probably not.
So to bring this back to football, we have to recognize factors beyond mere "supply" and "demand" that sets the price of labor for these players (or, phrased differently, the outside factors that influence both the supply and demand). In that case, we would need to recognize the relationship the NFL has with NCAA Football and its use as a de-facto minor / development league. The overwhelming majority of professional football players are groomed and farmed out of the NCAA system, and the earning opportunities of those athletes are limited by the constraints of *ahem ahem* "amateur" athletics . That is accomplished by both the NFLPA setting an age floor on being allowed to play in the league, as well as the NFL having an antitrust exemption on their monopoly with all major networks that stifles competition.
So it's nice to say "if you don't like what the NCAA is offering you then you can go do something else", but when the labor market for under-21 football has become warped by those anti-competitive factors it doesn't really hold as much weight.