That quote is only relevant when you look at it in the context of never, never, never, never give up on your entrepreneurial dream.
Trying something and seeing that their is no market for it or its just not working and continuing to work on it, is not the intent of that quote.
It's hard to know when you should quit and try another idea. But, quitting opens the runaway for the next better venture.
Also, each "failure" is a stepping stone to gaining the skills to make the next attempt easier.
If you look at any major founder they have had a history of starting small companies or failing a couple of times until they found the right business. They had the good sense to move on from their failures but they didn't quit.
At the end of the day its about maximizing your bandwidth. For someone with limited time due to family and kids, the bandwidth is just not there. So writing a book, maybe focusing on iOS apps, etc. might be the better approach or finding a niche that can accommodate a slow ramp up time would be the best.
I would love to know about others who have gone the iOS route. I'm thinking about focusing the next while on gaining iOS skills to develop Apps. The strategy would be to develop some niche apps and see where it goes. Even if it turns into passive income and I've gained new skillset, it might still be win/win.