Sadly, I can see Apple eliminating the SS model altogether. I don't know for certain, but I suspect the aluminium is cheaper to produce yielding greater margins (even with a lower selling price).
Think about it...other Apple products have started out with stainless steel, only to go aluminum including the original 5g iPod and iPhone. Even Apple's current website AW page doesn't show the SS model.
As watches and wearables become more popular, lower priced aluminum models will drive mass adoption. There's probably already anodized models in a rainbow of colours in development. Lighter, slimmer throwaway watches, all sealed with glue.
I hope I'm wrong and that the SS is here to stay. Apple's vision is so driven by efficiency and profits that if demand is tepid, it may look to go all aluminum.