I've recently acquired complete box sets of TNG and ENT on Blu-ray and have been watching some episodes on my brand new front projector (1080p, as 4K projectors are way out of my budget). TNG looks fantastic. It's clear they went back to the source material and did a proper restoration and post-production. It has never looked as good as it does now on Blu-ray. ENT looks...okay. They have not remastered it (it originally was mastered at 1080i for eventual HD TV broadcast, with early 2000s level PQ). Also, the first two seasons' effects were rendered at 480p, with the last two rendered at 720p. The final season was shot on HD digital cameras rather than film, to save costs. Ordinarily, this would look worse than the 35mm film used for seasons 1-3, but the latter has not been re-scanned, so the HD digital stuff looks best.
In each case, I got a very good deal on the box sets (a one-day Amazon sale at 75% off list for TNG and a very good, post-Brexit exchange rate deal for ENT from Amazon.uk--ENT is not available as a box set from a N. Amer. source as far as I can tell). For the price I paid, TNG is a steal in terms of A/V quality and ENT is a fair price. For either at list--I don't like them enough to pay full list (I can afford them, but there are few A/V purchases I consider worth list price--about 20 of the 1200 or so titles in my collection).
TOS and TNG were worth the studio's gamble on remastering from original source material for HD at the time the decision was made, based on their respective popularity. ENT was an easy task, though clearly comes up short despite being the newest material (if treated the same as TOS and TNG, it would be the best looking/sounding of all the extant series, but clearly there was no incentive to make such an effort). DS9 and VOY would, very best case scenario, cost as much as TNG to do it right--and probably more. No way that is ever considered a viable proposition if TNG is not proving a success (for the quality, and the price it sells for now, any fan of TNG with decent A/V HD gear should grab up the series--I've compared it to HD on Netflix and the Blu-ray looks way better). In another decade, when technology makes the transformation much less expensive, it might happen for DS9 and VOY--but the people with the original source materials better keep them stored properly. Otherwise, the age-induced degradation in the film or video source will offset any savings of cheaper transformational tech. The ability to make things look spectacular, even with less than pristine source elements, is rather impressive (I have a blu-ray of The Birth of a Nation, which I use in class on occasion, that looks excellent for its century old source and outshines much that is 15-20 years old). But such restorations are NOT cheap. For a movie, the justification is difficult at times. For a 170+ episode TV series, there better be some deep pockets.