I think the most likely scenario is that Google will make the present versions of all plug-ins free. But, they won't be doing any maintenance on them.
The technology behind the plug-ins will be integrated into Picasa, Picasa Web, G+, and other current Google products as appropriate. I do not think there will be any current Nik products that will continue to be sold. There are very few desktop software products that Google sells, SketchUp and Earth Pro are the most notable examples I can think of.
Just as a fantasy, maybe Google, Nikon and Nix have a grand plan to combine Picasa and Capture NX to create a Lightroom-like piece of software. Like I said, just a fantasy.
As far as Nikon goes, I would hope (and assume) their lawyer's drafted a good contract that ensures the development, support, and use of Nix technology for at least 5 years. Then again, Nikon's general lack of interest in developing software might mean that they overlooked this as a business concern and failed to secure their interests.