Earlier quoted context omitted.
It's a classic collective decision problem. If almost all postdocs at the same time decided to boycott closed journals, then this would work. However, in practice solutions are always failing whenever they are based on the idea that a majority has to voluntarily make the right move within a short time period, if that means smaller wins for people who do not make the move. To give another example of such a problem: If…
What if it's "kickstarted" with people pledging to submit papers to only a select few publishers once a critical mass of pledges have been received?
Good question. Finding ways to disincentivise defection would be key.
You might want to look up Mancur Olson's "Logic of Collective Action", which deals with this problem, generally.