There was 22 years between the first air date of that 70s show (98) and the year it was set in (76).
There was a 28 year difference for that 90s show. I never watched that 90s show, but I'm guessing the reason for its limited success has more to do with the quality and the nature of the current streaming environment than the setting.
This makes me wonder how many shows we consider classics only got popular because the came on after something else we liked and we didn't change the channel. When given the choice to watch them we just... don't.