They are available, intact, on DVD, on a thousand DVD Rips, and torrented around the world. They aren't going anywhere. They've had an official digital release, which in this day and age makes them immortal. I don't see what more you require. It's not as if all existing DVDs are wiped when the (hypothetical) Blu-ray release hits the shops.
DVD will eventually become obsolete, just as VHS has. My argument is that the original episodes needs to be available in whatever the -current- format of choice is, regardless of what previous formats they are available in.
Again, I don't understand where the big point of contention is here. We've already established that Paramount could take the original, standard definition episodes and toss them onto the Blu-ray discs without in any way increasing their costs or affecting the enhanced HD releases. So why not do it?