A lot of early chest thumping, but the logic is sound. As far as I understand, this isn't a full contract, only a two-year extension. Assuming it's somewhat palatable (and the 13% rumor is a great start), there's very little to lose.

If this goes through, the benefits are immediate and in two years the leadership starts negotiating again, but from a 13% better starting point.

If it fails, it will still be another two years before they open the contract anyway, there will be 13% less pay until then, and a 13% lower starting point the MEC will have to negotiate back. Not to mention, it will most likely require the replacement of the MEC (reference Delta and Southwest) and put in question the credibility of ALPA's ability to represent it's pilots. The incentive for any new MEC to reach a TA will be shot. Reaching a TA would be the largest threat to union leaders.