Flathatter
I think you will find that any kind of homogeneous world wide seniority system is already dead in the water.
Imagine, if you will, arriving for your final interview with a major carrier, long established etc. You are told that you have got the job, (LH/AF/BA/CX/QF et al), but would you pleae sign this waiver, that you will forgo any rights under a world wide seniority scheme since the outfit you are about to join is long established, has no intention of going bust and in any case looks after it's own with promotions from within, etc. etc. Would you take the job of a lifetime or walk away?
The only place a system such as you suggest might work would be in the lower end of the market, do you think Ryanair, for example, would go for it?
Never forget, as a species , pilots will eat their own young.