I was surprised they didn't scan the 35mm negatives in 4K at the time, but cleanup and processing time would be increased as a result of the extra resolution. But, yeah, blu-ray is the best we'd get and that's pretty darn good.
And some film stock wasn't the best quality, and the speed of some film had to be higher to work in darkly lit spaces - like some scenes in "Contagion", especially on handheld cameras, the result of that different film is more visible grain of the sort nobody likes to see, where 4K or higher wouldn't make as much sense (like doing a 16mm negative remaster on blu-ray, 16mm doesn't have the resolution - but it does have a wider color gamut than videotape or what a standard DVD could allow for (compression issues)). Season 3 onward largely got much better quality film stock, and sets were often lit differently to prevent the need for the type of film needed to capture any detail in dark settings (at the cost of more visible grain.)
Also, native 1080P upscales much easier to 4K than going from 480i to 1080P. I too would prefer DS9 and Voy being remastered even in 2K from the original 35mm negs, which are much richer in color and sharpness. Even if their reliance on CGI is much greater and more complex, requiring more work - especially if the original CGI objects and files no longer exist.
Lastly, 4K works best on a BIG screen at a several dozen feet away. For home viewing, there's really not that much gain unless one has a 70" TV they're ten feet away from. There's a website that has a distance calculator for 4K/1080p/etc, I don't recall it offhand...
