I don't know if it's fair to say that these shows were never meant to be seen in anything but standard definition television broadcasts. Roddenberry tried to get Jeffrey Hunter to shoot extra footage so that "The Menagerie" (i.e. "The Cage") could be released theatrically, and many of the cast had clauses in their contracts covering theatrical exhibition.
Mission: Impossible, the Desilu series which premiered the same year as Star Trek, actually did cut together a pair of episodes for foreign theatrical distribution (the second season two-parter "The Council" became Mission Impossible vs. The Mob).
That's a valid point, but I think even to audiences of the day, these were obviously not "feature films" in budget or scope. I thought The Cage would have been broadcast as a movie of the week with the additional footage to bring it to around 90 minutes.
And back then even movies were "one and done." See it in the theater and then that's it. Maybe it would be on TV once in a great while. Today, everyone watches stuff over and over on DVR and home video and with HD inches from your face, everything comes to light. Aside from a couple of space shots, all of the effects in The Cage are fine. Mostly laser effects, illusion dissolves and matte paintings. Generally, though, TV production at 5 or 6 days per episode on very small budgets, shortcuts had to be taken and a lot of them could be obscured by low resolution broadcasts. Even the amazing effects work on Space:1999 and Battlestar Galactica in the 70's is betrayed by hi-def.
The illusions are preserved in lower def. Do I really want to see the strings holding up the Flying Sub on Voyage to the Bottom of the Sea? I had never noticed the ear seams on Nimoy until the blu-rays. Now I can't unsee them.