well said yellow flag,
I hear a lot of argument pro the seniority system and you have pretty much covered them all, and to be honest i tend to agree with your sentiments. Having said that though, whilst i believe that seniority has a role to play i do not believe that it should be entirely "winner takes ALL"
Perhaps there is some room for some minor modifications to the system of work allocation, holiday bidding etc that may see some of the less senior get something rather than absolutely nothing. in order to achieve some sort of greater equity however we need to be sure that we do not destroy it for the majority in the process and also that we do not hand the system over to the company.
In a general sense, the reason that management do not like the seniroty system is basically a philosphical objection based on the fact that it gives the employees the majority of the power rather than the company.
Oh sure they have tried to undermine it with Demand days and courses and stuffing around with divisors and language allocation etc etc however, the system in general allows us crew to make the choices and not management and it Effing well nearly kills them
That alone is good enuf reason for me to want to keep it