I think you’re conflating two lines of thought that are seemingly against the seniority system, but one of them isn’t, and is actually a part of it.
It’s perfectly sensible that there are minimum qualifications for upgrade, and if they are established and applied in a consistent and transparent manner (X hours total time, Y hours time in type, etc.) that is not only right, but necessary. And it may be that a junior pilot meets these quals before a senior one does, and therefore receives an upgrade slot first. Maybe even at the time of hiring, if the company training program includes that provision, in the case of DEC’s. However, the senior pilot who later meets the quals and takes an upgrade, would then be bidding above the junior pilot if they’re bidding at the same base, etc. (Also, the position on the list and the bid award determine the entry into upgrade training, which can then be passed or failed based on demonstrated aptitude. It’s a red herring that the seniority system disregards aptitude.)
That’s different from an ill-defined nebulous “meritocracy,” which would allow management to pick the winners and losers based on economy, nepotism, etc. under a fig leaf. It’s also different from someone entering at “commensurate experience” at a seniority level above pilots who have already been paying their dues at their company under the pre-agreed terms of their contract. And this is not right.
As far as people choosing to remain in the right seat, that’s an exercise of their option to bid anything their seniority level can attain (or less) based on the multitude of factors in their life. You say that seniority system proponents fail to mention that, but here it is; and I don’t see what part of our argument is undermined by it.
Also you note people’s entrenchment in their position, and it’s certainly human nature to be entrenched in argument positions, especially when they’re self-serving, and/or confirming of their biases. Like a senior pilot favoring seniority, or an unemployed pilot being against it. But I’m not so entrenched, and as I said before I’d be willing to honestly entertain a national seniority system vs. a company one, for example. But only if a way can be devised for the transition from one system to the other, that does not violate existing pilots’ contractual agreements!