Some of you (maybe you're very young) are getting your time periods mixed up. There is no way that Brenda, Janet, or Sandra are 'seventies names'. My aunt born in the 40s was Janet. I would say it was a 40s/50s name really. I was born in the 60s, no other girls at school were named Brenda, Janet or Sandra, probably the mums of 70s born girls had those names.
I would say 70s names were more like Claire, Sarah, Louise, Nicola, maybe Tracey and Karen (although those two had been a lot more popular in the 60s I'd say) . Names like Emma and Charlotte were starting to come in although they really took off in the 80s.
I don't know which names will never come back, I'd have said Hilda, Winifred, etc. but I've seen those suggested on here a few times. I'm constantly surprised as I would never ever have guessed that Arthur or Elsie would come back.
Maybe Gertrude might not, that's horrendous, but then someone the other day on here said they really liked Trudy and I suppose they might put Gertrude down on the birth certificate maybe.