"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.
Actually, if I think about this more, then the brick analogy is very useful - if you see someone (or better yet, some whole industry/niche) beating themselves on the head with a metaphorical brick, then this means that they have a very, very important problem and most likely if anyone came to them and told that they have a solution that's simply unpolished but solves most of the problem without hitting your head with the home made brick-solution then the expected response to this description would be "shut up and take my money!".
It pretty much implies that you're attempting to overtake existing competitors who have found product/market fit, where people have options to fill their need but do so grudgingly because of their UX quality. Doing that can be a solid business plan, but it's very, very different from the type of new-product-discovery startups that Reif Hoffman was talking about.
If the people are paying for an existing crappy product with poor UI/UX, then that illustrates a valid need and building an easier-to-use product can greatly increase the size of that market.
However, if currently people are not eager to pay for that need, then I'd wager that simply providing UI/UX/ease of use won't change that. If their need isn't sufficient to put up with the minor bother of sucky UX (at least for 10% of most needy part of the market), then that need isn't also sufficient to put up with the much larger bother of paying you adequately.