YOu simply can never go back--what made originals appealing were due to a combination of factors unique to a certain point in time and in the viewers' lives. That is why so many resurrections/reboots fail. Viewers think they would love an old show to come back or have a reunion but it never is the same and they are left feeling like it would have been better just to leave the series dead and have that pining but it never being fulfilled.
It also doesn't help that they usually just have the old cast as props and not the focus.
Everything is better in retrospect. I remember watching the A-Team in the mid 1980s and being absolutely glued to the screen.
Now I see reruns and I spend half my time cringing. After 20 years have passed, you remember a tv shows great moments that might've occurred every two or three episodes. You forget that there were about 90 minutes of crap that you quickly forgot in between.
In regards to Dallas, for every "Who shot J.R." there was also a "Bobby in the shower".