Why were the stuntmen so damned OBVIOUS?

"No high def TV back then"

People these days underestimate the quality of TV "back then." They either weren't around back then or they never had good reception. In reality, the quality of NTSC broadcasts on a standard resolution CRT TV was quite good, assuming perfect reception. For example:

https://i.imgur.com/xu62D7t.png

That's a picture of my cheap 12" Samsung B&W TV that was made in 1984, with an episode of The Twilight Zone playing on it. The TV only has an RF input so it's the same as it would look with an over-the-air NTSC broadcast with perfect reception.

"color was a novelty"

Color, or lack thereof, doesn't matter. The B&W portion of the signal ("luma") has drastically more resolution than the color portion ("chroma") anyway. You don't lose any details with a B&W TV; you just lose the color information.

"safe to say most people didn't notice."

The stuntmen are very noticeable, as long as you have good reception and your TV isn't in need of repair or drastically out of adjustment.

If that. Some of the engineering sets were cardboard and they would flex and crease if someone touched them.

Yes, that's another very good point! And we probably have (dare I say it?) the third season to thank for that. Without it, there wouldn't have been enough episodes for "stripping" the show five nights a week, which is how syndication generally worked back then.

And syndication is where Star Trek really became a phenomenon.

LOL

Watching old The Wild Wild West episodes it is impressive all the stunts Robert Conrad did himself.

I remember something kind of funny in one of Lee Major's scenes as Heath Barkley. They used a stunt double for a fight scene and his boots were a different color than Heath's! The part with the double goes by fast. It's hard to notice. But I did notice and replayed the scene. Back in the sixties, audiences couldn't do that. So it was no big deal.

I've also freeze framed some fight scenes on Bonanza and you can clearly see the stunt man. It was easy to notice in a scene with Hoss even with only the back of his head showing. There weren't any stunt men as BIG as Dan Blocker.

That's my favorite DS9 episode as well (and if we'd known it was on the remastered original-series set, we wouldn't have bothered buying an entire season of DS9 just to get it). And I love that joke!

A minor correction: It was Paul Baxley (Lt. Freeman and Shatner's stunt double) who was mistaken for Kirk (although Eddie Paskey, AKA Lt. Leslie and Shatner's stand-in, was also in the borrowed scenes. My fuzzy recollection was confirmed by Memory Alpha (near the end of Act Three, next to the photo of three TOS Klingons): https://memory-alpha.fandom.com/wiki/Trials_and_Tribble-ations_(episode)#Act_Three