And TNG can't be shown in HD because it doesn't exist in HD. How many more times are we going to play this game?!
We all know it was finalized in 480i back in the 80's/90's. But that doesn't stop it from being broadcast by HD providers nor does it stop it from being watched on HD displays. As is.
Whether or not you can enjoy it under those limitations is up to you.
But but...arrgh, Those outlets you mentioned are exactly the same as watching it on DVD. Yes, it's good enough to watch but its not good enough for a High Definition stream. It does not have the data required to carry the detail. It's like having a size 8 foot in a size 12 shoe. Sure, it fits, but it's damned awkward walking about in it.
IF TNG could not be seen in higher quality, I would be against digitally restoring/upscaling it for a Blu Ray release. However because the means exist to add detail to the picture that was there when it was filmed but lost in the process to put it out to the masses then by all accounts that detail should be restored and broadcast in the way it was originally recorded.