Future-proofing is what it's about! Eventually we will move out of HD to Ultra HD, 4k, etc.. so if we leave Ds9 and Voyager in SD they really become more useless as time goes on.
UltraHD / 4K yet needs to prove there is market demand. On a average sized flatscreen you'll hardly notice the difference and the presentations I saw at the IFA 2013 mostly had the HD Screens manipulated to look "bad" next to UltraHD displays.
But, indeed, future-proofing is necessary.
However, nobody seems to consider the possibility that DS9 may be the first next-gen Star Trek series that theoretically might
be re-formatted into 16:9 (despite the fact it was shot in 4:3 and not "protected" for widescreen extraction until seasons 5 or 6) to appeal to larger audiences.
Regarding the amount of CGI-"cleanup" I mentioned earlier, reformatting the series into 16:9 could reduce some of that cleanup work and therefore reduce part of the remastering costs.
The one thing I just can't help wondering is this: ENT on Blu-ray could have been released any time between 2008 (end of Blu-ray vs. HD-DVD Format war) and 2012. Especially given that the VFX were "only" HD ready.
Why now and why make it compete with purchases of TNG in HD?
It almost could
seem like CBS is using ENT as a means to get us interested in Star Trek in 16:9 widescreen...
This is just mere speculation. But I wouldn't exclude the possibility that it might be DS9 in 16:9 or no DS9 in HD at all.