I'm fairly hopeful that in 5 years or so when they'd start remastering DS9 it will be cost-effective to do the CGI work though.
Where is this 5 year number comming from?
Guessing + bad phrasing. What I actually had on my mind was five years until they'd start
releasing DS9-R. Basically I expect the release of TNG-R to take at least three years (amount of work, spacing the releases to sell better as captrek already explained, etc.), and then there's going to be some lead time to ramp up work on DS9, just as there was between TOS-R and TNG-R.
Let's not get ahead of ourselves, though. I'm fairly optimistic that they're going to do a DS9-R eventually, but it's definitely far from a sure thing. Compared to TNG, the number of non-stock effects shots they need to tackle for DS9 is astronomically higher, as is their average complexity, and the amount of CGI work necessary for them. And it's not just space, either; there are entirely new challenges like doing Odo on a fairly regular basis.
DS9 is the product of a post-production department that had most of TNG to train and oil its machine, and as a result it pushed the envelope far above what TNG did. The model photography shots in DS9 are the most sophisticated ever done for a hourly TV show (just look at that one mirror universe episode with the Defiant dodging around the station and the Klingon cap ship; I read a Making Of article on that once that claimed it was the most complex motion control shot ever done for TV;
edit: see
here and
here) and some of the things they did with CGI in later seasons were nothing short of breathtaking as well. It takes giant balls to want to play in that league, and you need to to redo it for HD and do it justice.
On the flipside, I think Voyager is actually easier again. To be sure it's harder than TNG too, and Voyager also had some brilliant and complex shots here and there, but you don't have Odo, you have less fleet battles, you have more shots again where just having a model of the ship gets you most of the way there, etc.
I'm actually kinda fearing they might skip DS9 and go directly to Voyager since it had better ratings and DS9 is just so hard to do.