As Saltaire said, senority isn't the most important thing . . . it's everything.
This is why you have some relatively senior pilots bidding FO positions rather than Captain. Why? Since their job security is not the issue, other things such as having more influence in their schedule, where they want to live, what kind of flying they want to do, or they really like the type of airplane. I've know a few fellows for whom money is not the major issue-- one's wife was a highly sought after attorney, he just went to fly to get out of the mansion.