I would argue that in legacy carriers, the pay is generally higher because unions are strongest there! Whatever way you look at it, no airline - when left to itself - pays more than it absolutely has to for a pilot.
I would say that your point about hiring countless DEC's is moot as:
1) this is the potential situation with F/O's and does that happen in any half decent carrier, legacy or not? The majority of the time, no.
2) once any self respecting company has spent the money to interview, sim-check, employ, train and type rate a DEC they will hold onto him - they needed him in the first place.
There are of course unscrupulous companies who already do this to F/O's in particular, but you will never change that. And besides the companies I'm talking about, those with unions, will be unable to hire and fire DEC's (that they've now spent a lot of money training)...that's the point of having the union there!
I have a problem with the prevailing attitude as well when pilots spend their lives complaining about management who line their own pockets, don't care about employees, are entirely motivated by self-interest etc and then just come up with the 'well it's your bad luck, I'm okay so I don't care about you!' when pressed...how selfish can you get! What's the point of having managers when we're all quite happy to shaft each other given half the chance?!
And while there is no economic benefit to the company, what about the economic benefit to you when you find yourself in the situation? Would you see things differently then, when faced with a 50% pay cut, or, of course, being free to move to the other side of the world - where, it seems, the streets are paved with gold, desirable jobs are plentiful and your family will fit immediately into life, loving it and supporting you. How will your wife and kids react to you saying 'Surprise! we're moving to Saigon!'?