Other way around then...
If you were the FO ready to be upgraded but constantly pushed down by DEC hire, would you be happy?
Life isn't fair, but seniority comes as close to it as possible. I think. DEC but may have its place in expanding airlines where time to command is short as it is. But it'd certainly piss off long times FOs in a mature airline.
I have tried both seniority and non-seniority, I prefer the former as I know what I can expect. Bankruptcy is always a risk, but so is base closures and other features of "flexible airlines" which can just as well make your life a missery.