I guess what matters in a problem is size * solvability. Solvability may be variable, but bigger problems do necessarily mean more room for improvement.

Even though it doesn't account for solvability, I still think it's a good quote. Most people become very accustomed to the way things are, even if they're bad, and never think to change them. This quote reverses that mindset, by getting you to think in terms of opportunity. Once you get over that hump, then you could think about solvability. The real point is that the hump is very important to get over, and this quote is pretty effective in getting you over it.