"worker-owned consulting companies"
There are many organizational forms. I wouldn't choose a company. It should be a cooperative, as described at http://www.sba.gov/content/cooperative . "Not all cooperatives are incorporated, though many choose to do so." And "Democracy is a defining element of cooperatives. The democratic structure of a cooperative ensures that it serves its members' needs."
"Can you explain this claim? What prevents any employee from leaving and selling their labor to another willing party?"
Sure. You mentioned Keynesians. Quoting the Wikipedia section about cyclical/Keynesian unemployment: "With cyclical unemployment, the number of unemployed workers exceeds the number of job vacancies, so that even if full employment were attained and all open jobs were filled, some workers would still remain unemployed."
In that scenario, there are few willing parties to sell one's labor to. How is that not structurally similar to a monopoly? An employer in that situation can abuse their monopoly power, and take advantage that the switching costs for the employee to get another job are so high. In short "you take a 5% cut in pay or I fire you and hire the next person who walks in that door." It doesn't even need to be said: "you will take a 5% cut in pay" implies "or you'll have to quit and find another job."
We don't need to be in a Great Depression for that to happen. Or do you think the 2009-2010 spike to 14% unemployment rate for Michigan was all due to people deciding to stay unemployed while holding out for higher paying work?
Using a similar calculus to your model, the switching cost for an employee includes [1] the difficulty of finding a nearby job, or moving and feeling uprooted (and finding new schools, new job for the spouse, etc.), [2] potentially being called a 'complainer' or 'quitter' or labeled 'unable to handle heat' by members of the community or black-balled by industry, [3] the lost wages/opportunity cost between quitting one job and starting the next, [4] the basic stress of having to get up to speed with a new job, meeting new people, and understanding the new social environment, [5] the emotional impact of looking for a job and getting a bunch of 'no's (My Mom got her EE degree, as a 50 year old woman, and tried looking for a job. The many 'no's she got became quite discouraging. People may stay with a job, with its external torments, than deal with the internal.)
You may object, and saying that if a person stays after a 5% pay cut then it shows that the job was priced above market rates. However, I would consider that practice an abuse of monopoly power.
Further, there's a Gambler's ruin issue to quitting, with the employer taking the role of the casino. It might be that a person has a job lined up, moves across the country, only to find that the position is soon no longer there. If that person's unlucky (as what happened with my Dad when I was little), then that could happen twice in a year. (We moved in with his parents for a few years while my parents built up savings again.)
When someone quits, they take the admittedly small chance that they may end up sleeping in a car or other situation drastically worse than what they would had had, should they stayed. While the likelihood that the employer will have correspondingly large negative impact when an non-key employee quits is significantly, even laughably, smaller.
This too makes the employer/employee relationship more unbalanced, and so open to abuse by the side of the employer.