If seniority went it would be a bloodbath in this country as there would be expat pilots all over the world bidding for command slots. Not to mention pay. I'm sure Cathay and Emirates drivers would be happy to take a small pay reduction to pick up a QF or Virgin command based in Syd or Mel.
Unforunately the market in Australia is to small to open it up as such. It would be absolutely fantastic if you're an airline however, you'd have an almost unlimited supply of very experienced pilots from all over the world under bidding each other to get jobs. To consider that option as a pilot based in Australia is unthinkable. Yeah it works in Europe only because the labour market is in the pilots favour and they can pick and choose once they get experience. Asia has a shortage of drivers and nowehere to get experience guys from locally, so they have no choice.
Your argument also doesn't hold a lot of water in an airline environment when dealing with high experience candidates (as you would be for say advertised DE command slots as you have a great deal of difficulty differentiating them.
If all the candidates have the aptitude to get the job (becuase they do as they have passed the initial screening), and they all are good pilots (and they will be because they have to pass all the cyclical training ) then how the hell do you start differenitaing between candidates. Also if considering pilots from other major airlines how can you verify his training records? However with your own guys you review his entire career with a click of a button.
One thing seniority does is rules out "jobs for the boys". You will not get a command just because you are best mates with the chief pilot. For the airline seniority encouarges loyalty and the captains you are getting are of a known quantity.
And even with the seniority system you still have to pass command training. It's not like your number comes up then you get a job. Your number comes up and then the hard work begins.