"then you're obviously not entering/creating a new market, nor you are attempting to find a product/market fit, are you?"

Yeah, and if you were entering/creating a new market, usually people do not understand why they need the product you are creating and thus the original analogy still does not work.

Overall I just feel the brick analogy is not great because there are no low hanging fruits anymore. If a problem is so painful, there must exist a competitor OR a home made solution.

Agree with the rest of your statements.