Compare those episodes to any random episode of Deadwood
and you'll see what I mean. Broadcast TV in the 60s (or today) wasn't allowed to do dark like cable does dark nowadays, not even remotely close.
Of course my point is not that Star Trek
should be Deadwood
. That's exactly what I don't think should, or needs to, happen. DS9
is about the right shade of darkness for Star Trek
on cable. But in that context, it will look almost pollyanna-ish in comparison with the other shows on the same channel or just a click away.