A system a la Virgin where pilots rotate between three priority groups, with seniority applying within each group, seems like a decent compromise (although I don't actually know how well it works).
How about those who want to rotate seniority do it between themselves and leave the ones who don't with the status quo.
I'm a serial Blindline holder on longhaul and seem to get at least one weekend off a month generally. As I see it, JSS will mean in theory I work a bit harder (difficult to see how and not hit limits) but I'll get more choice over what I do.