The top thing I've learned in the last 25 years of systems engineering and architecture is that if I'm being asked to modify an existing system as opposed to replacing it, I need to spend an awful lot of time learning why things are done "the way we've always done it". In the majority of cases, things are done the way they are because there are good reasons to do it that way that have stood the test of time. There occasionally are instances where things are done in a way that is stupid that reflects a stupid dicate of current managment, but stupidity generally doesn't survive regime change - the people doing the work know it's stupid and they convince the new management there's a better way.
Applying that wisdom to the airline and the 1500 rule, I have two observations:
First, there didn't seem to be any rational reason to apply the new rule. That's ok, it's a political mandate. Again, I am not speaking of crew rest or other parts, just the ATP requirement for part 121 FO's.
Second, I don't know anywhere near enough about the airline business to presume to tell them that their HR system is wrong/bad/stupid/etc, let alone to suggest changes. The seniority system is a teeny-tiny part of the overall airline business model, and changing it willy-nilly would almost certainly have significant consequences elsewhere in the system. Given that the profit margin in the airline business is already sucky, I'd be hesitant to make any changes without an awful lot of careful consideration.