Watterson was definitely a workaholic when he worked.
To be fair, non-internet cartoonists make and sell cartoons. Internet cartoonists make and sell merch, they then make a cartoon in their spare time in order to promote that merch, so Onstad has some extra duties. I feel kind of guilty as a webcomics fan, but I don't have much desire to wear T-Shirts that say to the world "hey, I'm *really* into the internet these days" so the artists don't really get my support.
I do wish Watterson would start doing graphic novels though. I think Calvin & Hobbes ran as long as it should have... it was such a rigorously defined little world, only two full characters, and half a dozen minor roles, tops, that it could only have gotten stale or had to change significantly. He wrote in the past about his disdain for comic books, and looking at the state of comics and comix in 1995 or whenever he was writing, I can understand his point. At the time, comics, both mainstream and underground, were overwhelmingly about the adolescent male id. Things are different now, though, and I think the comic book industry that puts out things like _Blankets_ could really do justice to Watterson's talent.
I would anticipate his objection to modern graphic novels as this: newspapers are a populist medium, like radio or broadcast TV. You catch people of all walks of life and you can catch them by surprise in a newspaper. A graphic novel will only sell to people who are plugged into the subculture, until you get some sort of external press, like the New Yorker busting a nut over Jimmy Corrigan. The artsy comics market has the trappings of both underground and snooty culture, and it seems like those are two audiences he doesn't much care for.