What characteristic defines an invention that becomes commercially successful, thus disqualifying it as true Chindōgu?

Answer

It becomes useful and is commercialized.

An invention that becomes useful enough for people to buy, or that is patented and sold for profit, technically violates the core spirit of Chindōgu, as exemplified by the selfie stick.

What characteristic defines an invention that becomes commercially successful, thus disqualifying it as true Chindōgu?
inventiondesigncultureuselesschindogu