I doubt that blu-ray sales are as big a factor as some people think. The way I see it if CBS wants to keep making money off of Star Trek not just from home media but also streaming, syndication, and what ever else the future holds then they need to "Future-Proof" the entire Star Trek catalog by remastering everything because at some point HD will be the normal accepted format and SD will be considerably less valuable.
That's it exactly. The studio is looking at the big picture, not just BD sales. They want to continue to milk money out of these properties. DS9's remaster is virtually assured, as is VOY's.
Yes, there's more CGI in those series than there was in TNG, but the restoration teams have probably learned by doing TNG how to do things even more efficiently and will apply what they learned to the other series to help cut costs and reduce the amount of time it takes. I imagine they're learning as they go and will learn from their mistakes as well as their successes how to make a DS9 remaster as efficient as possible.