Considering last year the two recovered Doctor Who stories that were B&W and barely SD briefly topped the iTunes charts, don't write off older formats just yet. Digital downloads offer a bigger library than possible before.
Doctor Who is a whole different beast though. Those were episodes that people had thought were lost forever, so to see them in any quality is a rare thing. I'd bet a good number of classic Who fans didn't even see those stories when they aired on television, so them being released was a major thing. I'd say that Doctor Who is the exception rather than the rule.
Of course not that it'll factor in to it, is that ST has already earned hundreds of millions if not billons already.
That's exactly why CBS wants to keep being able to make money from this franchise. They've probably pulled in hundreds of millions from this brand and I'm sure TOS and TNG restorations will pull in huge numbers over their lifetime as well.
CBS isn't getting any new Trek any time soon (unless they surprise announce a new series), and considering that the well for DS9 and Voyager is probably getting dry, I don't see them just leaving it that way. Unless getting them to some form of HD is going to cost an obscene amount of money (and if they don't do a full on TOS/TNG-like restoration that likely wouldn't be the case), CBS will want to do something with them.