They're great, thought-provoking dramas, but great sci-fi?
As opposed to, say, "Miri" or "Bread and Circuses"?
Don't get me wrong. I just rewatched all three seasons of TOS, and it's still my favorite Trek series, but let's not pretend that it was on a whole different level of scientific plausibility than TZ. If one is so inclined, there are any number of popular TOS episodes that can be made to sound just as silly.
A giant space amoeba? Abraham Lincoln versus Genghis Khan? A transporter beam splitting Kirk into his good and evil halves? A copper-based Vulcan cross-breeding with an iron-based human . . . ?
Both series had their flights of fancy.