Still, with what's been found in the past couple of years, and the news that Chuck's Wheel was pretty well wiped out (which I now happen to believe), you can never tell for sure what still exists and what doesn't because new things are being discovered all the time.
--As far as what exists and what doesn't, I tend to take the view that we should say something is "lost" rather that "destroyed" or "wiped out". Simply put, as Ian said, new things are discovered all the time, and we don't know what may exist in a vault somewhere. Just look at the 1972-74 episodes of TJW--for years, we assumed they were gone forever, but we were wrong. They ALL exist, and we were pleasantly surprised to discover this in 2000.
Additionally, just because the networks no longer have copies of these shows doesn't mean they might not exist somewhere else. For example, the 2/21/54 episode of What's My Line? is listed by G-T as "lost" and is not in GSN's archive. Yet, the show is available on the public domain and can be purchased from Shokus Video. The reason is because even though the master kinescope is gone, Lucille Ball, who was a guest panelist that week, had her own copy.
Unfortunately, a lot of lost shows may very well be gone forever. We should never stop searching though and never give up trying to preserve these classics for future generations of fans.
-Flea