The most important thing seniority does is protect T&C, without it it is much easier to find people willing to fly for less because of a DEC position. One of the reasons T&C are so much better in the US right now is the fact that everyone is union and seniority.
If a company requires captains and can't upgrade FOs the DECs that are hired should be last on the seniority list. Yes, it is hard to start over after your company fails, I have done it two times myself. It is even harder to see qualified FOs being by-passed for upgrade, while DECs were getting hired above them. Just my opinion.