Old films can certainly look great on dvd, if they still have high-quality 35mm elements. There are some old films going back to the early-1930s that look absolutely stunning, if top-notch source material survived and was used.
Do you know what looks bad, though, on modern TVs? Television shows from the 1970s/80s that were shot on video-tape, like sitcoms and variety shows. Also, in the 1980s and maybe even early-1990s, a lot of otherwise nicely 35mm filmed tv-shows unfortunately used videotape for editing their master copies. Thus the masters are rather soft for high-definition sets, and look less impressive than a lot of older filmed tv-series from the 1950s/60s/70s.
You are right, if they have the material to work with they can do a lot. I'm a huge fan of Hitchcock and what they've done with many of his movies is amazing.
Do you know what looks bad, though, on modern TVs? Television shows from the 1970s/80s that were shot on video-tape, like sitcoms and variety shows.
I've noticed that when flipping through channels and come across a show from when I was a kid only to see that it looks worse than ever.