I agree that you've identified one of the worst effects of seniority on all employees. Take, for example, the US East guys - with decades of experience. They would have, in theory, been able to go elsewhere in a system based on experience (not "merit", which we've discussed, but their actual time flying a particular type) without starting over. The same for FA's and GA's, etc.

The reason for the existence of these 'lockups' is twofold - first, it keeps the dues flowing into the unions coffers (staff can't easily leave) and second, it also affords the employee some "protection" in terms of job security. What that protection has been worth to folks at East is debatable.

IMO, the union would never allow this to happen. The first one to do it would open their members up to an inflow of others... and conversely, if only one did it the members of the union who made the change would be the only ones impacted - and them for the worse.