Having worked for two airlines that hired DEC, this is also a minefield. Unfortunately training isn't the same across the board, and whilst I am sure that a DEC from TCX would have been trained to a decent level, that isn't the case everywhere. I have flown with DEC's where it really wouldn't surprise me if they had never held a prior command. (And yes, that is the new airline's fault for hiring after a 20 min sim ride, and minimal background checks.)
I also agree that seniority is archaic and stifles a free market. I truly believe the current situation at BA wouldn't happen if the senior pilots could easily move on to similar or better terms,
I like the idea, that once you have the qualifications for the position, you can then apply and join the appropriate list. No DEC, but rapid cmd so that the individual can be properly assessed. A captain moving would have to speculate to accumulate. The airlines would have to ensure that the process is fair and transparent to attract such experience.
Base moves, fleet moves etc, free to join a list, but then frozen after a move for a period of time so that the individual has a serious think about what they actually want.
My 2c anyway. But I know it will never change. Pilots are still very much stuck in the old apprenticeship mode... "I had to put up with S**t, so I will now make you put up with the same..."