They continue to improve and expand the line so rumors of Sig moving away from the 556 seem baseless. The suits at Sig have long spoke of becoming a rifle company that also makes pistols so the expansion of the line with the 516 series seems natural.
It would be counter-intuitive to invest in the R&D, tooling, marketing etc. to launch, improve on and expand the 556 line and then drop it.
I saw a bunch of new 556s at the gun show over the weekend that have the rear machine cut and screwed-on sling eyelet. It seems they are trying to improve and refine the 556 rather than move existing inventory.