When it comes to GPU launches it's ultimately TSMC who controls when something launches. Even though Sea Islands is 28nm, AMD has to be sure to pace themselves so that they don't have too much of a gap between that and their 20nm GPUs.
I think it's safe to say we're off the yearly GPU cadence right now, at least in the two generations per node scenario.
It would make more sense to have a year of the older generation and 20+ months of the later, faster cards instead of say 16 months of the older gen and 16 months of the newer gen.