IMO you shouldn't build a company from a bad idea, but you can in your spare time try to validate an idea you're uncertain about. But don't quit your job at Google.
"Of the 26 companies that I consider realized or effectively realized in my personal track record, 17 of them made complete transformations or partial transformations of their businesses between the time we invested and the time we sold. That means there a 2/3 chance you’ll have to significantly reinvent your business between the time you take a venture capital investment and when you exit your business."
Source: http://unionsquareventures.com/2007/11/why-early-stage.php
"Here’s an interesting breakdown of the “transformers” versus the “stick to our plan” investments in my personal track record.
Greater than 5x – 11 total investments – 7 transformed, 4 did not
1x to 5x – 10 total investments – 6 transformed, 4 did not
Failures – 5 total investments, 1 transformed, 4 did not
Unrealized Union Square investments – 6 total, 3 transformed, 3 have not"
For example if someone starts a dating site but later transforms into a general social network, I would call that a complete transformation.
However if you start out making a ruby IDE but later realizes that real money is in MMORPGs or whatever, well you might have as well as waited.
Consider: The variant of (Poor People) + (Profitable Idea) = (50/50 Success) and the variant of (Great People) + (Unprofitable Idea) = (Failure). If (Great People) it is far easier to get (Profitable Idea) then to change (Poor People) to (Great People).
To use your example, if you're making a Ruby IDE and no one pays you, you will never be profitable. If you make a Greate Ruby IDE and no one pays you, you will never be profitable. Discovering that MMORPG's are more profitable one day, you build one, and people pay you, you might be profitable. You could consider the time you spent on your Ruby IDE a waste, or you could consider that you've learned something about software development as a business. It is far easier to change the idea then the people.
I also submit that it'll be easier to quit Google when you've discovered your home run idea than to change your entire business.
You could not be more wrong on this point. Being at google is a recipe for complacency. Quitting your job is incentive to actually figure out how to build something good. Because it's do that or fail.
The successful ones that are a result of an initial good idea are a strict subset of the ones that choose a promising market.