It didn't hurt...
That a number of episodes were written by professional SF writers. The two examples you mentioned ("Doomsday Machine" and "City on the Edge of Forever") were written by Norman Spinrad and Harlan Ellison, respectively.
Recently, some original Star Trek episodes were "tweaked" with CG and released to TV. I thought this was done sparingly, and with some care, and I thought it worked pretty well. I don't mind this sort of thing as long as the originals are still available.