Are you talking about a wholesale disruption of organized labor at the airlines? Unlikely to happen, even as stacked against unions as the RLA is. A national seniority list? Also very unlikely.
The RLA is designed to encourage nearly endless negotiations. Airlines don't have to get rid of their unions; they have deeper pockets than the unions and can generally negotiate for years while waiting for the most advantageous time to update working rules and compensation. Getting released for self-help is not impossible, it's just a very onerous process. If unions (or union members acting on their own) do anything that even hints of unsanctioned job action prior to that release, the union can be crushed in a lawsuit.
It's why you don't hear about sickouts or walkouts very often here in the US compared to the European airlines. They're illegal and the union will run out of money very quickly if they lose a lawsuit over them. Why sign a contract quickly if you can play for time, wait for an economic downturn or a favorable labor market (from management's perspective), and the whole time the work groups are taking a pay cut in real dollars, even if you pay out retro.