I don't accept your argument, but even if I did, the article still has merit.
The opposing philosophy is, "drop everything else and go full speed ahead on your product". Ok, do that. Your likely outcome is still bad. We talk ad nauseum about the successes in our field, but fool ourselves about the failures. The failure model isn't Kiko. It's the 1,000 PHP apps that we've never heard of, because they didn't get off the ground after they used up the runway they got from their founders 3 month sprint.
No matter what you do, the odds are very much against:
* You making enough money with a web app after a 3-month sprint to eat.
* You closing a round of funding of any sort.
On the flip side, if you do your product "part time", the odds are very good you'll still be nurturing your product 3 months from now, or 6 months, or two years. The "lack of focus" hurts your chances, but the sustainability helps your chances.
Also, let's just be clear about something: DHH didn't write the best web framework in the world. Rails is nice, but it isn't a prohibitively awesome technical achievement. If you set the technical bar at "Rails 1.0", most of the programmer posters here can clear it. Find another excuse not to take a swing. =)