It looks a lot better than it did ten years ago. People like Youngman or Walker or Browne or Young and Lebrun put in twice the effort for half the effect. I'd wager that most of you can remember a good Doonesbury series much more readily than you can remember any Beetle Bailey or Blondie from the last week. Plot isn't enough, mind you. There's a difference between 'ability to sustain a plot' and 'continuous use of only plots'.
Most strips switch between gags and plot, and they're good like that.
This is a theory, but I think that the numbers back me up on this. Syndicates want cartoons with jokes that can be understood by someone with no foreknowledge of the series. Peanuts , Beetle Bailey , Garfield and Cathy are good examples of this. All of the background that you need to understand an individual cartoon is contained in the cartoon. Problem is, there's no real correlation between this and the quality of a cartoon.