I'm not sure it matters much how many actually watch. A lot of PBS stations are just looking for cheap programming for a weekend evening when the potential audience is fairly low. I bet Lawrence Welk re-runs from 40 years ago are very cheap.
It's likely a combination of cheap programming to get and viewer donations. Many in the over 75 demos vote with their money. They tell their PBS affiliate what they want when they make their contribution. The Icy Hand of Death hasn't come for all the Welk fans (yet).
PBS also airs content aimed at the Woodstock generation as well(Music, Counter Culture, Vietnam, etc..). Nothing wrong with that...