But we don't have an individual choice to leave the union
I can't think of anything stopping anyone from forming another union and with enough support, go to the airline, start negotiations and, if necessary, take industrial action.
The huge discrepancies in pay and conditions between junior and senior crew is retarded. Yes, earn less and work harder while you are still new at it, just like in any industry. But there should be a time limit on those lesser conditions, not a condition of someone in front of you giving up their seat.
It will be a hard fight, though. If you just ask for more with no reduction for the current senior crew, the company won't like it as it requires them to pay more overall. If you suggest taking from the senior ones, the current union (run by those seniors) will do *anything* to stop you and others joining your cause.