Child, as I explained in a prior post, the big problem is not a matter of contingency plans. It is the fact that most NFLers have such short careers. This is why owners have had so much leverage over the past 30 years of labor negotiations. Setting aside a rainy day fund won't change that.
It can only help. In a war of pure attrition, sure, the owners will always win. But if the players are better equipped to handle the loss of regular season games than they have been in the past, then they'll have more leverage because nobody--including the owners--actually wants that to happen. It certainly won't give them the advantage in the negotiation, but it will lessen the disadvantage somewhat, hopefully to the point that they can get an extra concession or two out of the owners. Every little bit helps, and when the message (save some ****ing money for a change) is a good one in its own right, it just becomes a net win all around if the players listen.
Also, the average player who makes a week 1 roster as a rookie is 6 years. The average career length statistic is artificially deflated by the constant churn of players at the very bottom end of rosters who play maybe a handful of games over the course of their entire career. If you took a snapshot of active NFL players at any given time, and then took all of their eventual career lengths and averaged them out, you'd get a number much closer to 6 than 3.
Dramatically oversimplified example of this dynamic: imagine you have a union that represents 10 positions. 5 of them will stick around for 10 years. 4 of them will stick around for 5 years. The last worker gets replaced every 2 months. Over the course of 10 years, you'll 5 guys in the 10 year bracket, 8 guys in the 5 year bracket, and 60 guys in the one week bracket.
Based on mean or median, the average career length in this group is extremely short, as 82% of workers only stick around for 2 months. But if you took a snapshot at any given point in time of the active union membership, 90% would have career lengths of 5+ years, and thus the majority of active workers would benefit from holding out for a year to get a 40% pay raise.