I personally don't have any hate for Coin or what they are trying to do; I hope they are successful. But it's not something I would buy.
Both my debit card and credit card are chip-and-PIN cards, so I can't replace this with card, and none of the rest have magnetic stripes that anyone actually uses (the few cards that do have stripes, like my Costco card and driver's license, also have my picture on them and I can't replace them with a generic card).
This will be super useful for some people, but not for most people.
Not to quibble, but you have some contradictions lurking in there. On one hand, you're saying that, like Uber, Coin solves a problem for a small subset of people. OTOH, you're saying that they are addressing a big market.
It's an important distinction if you're making the argument that the parent's personal need for the service doesn't matter. That is, if this is supposed to be a mass-market product, then "random" people (including the parent and others on HN) opining about their need for it is much more relevant than in the case of a niche product that is intended for a small audience.
In any event, "small subset" vs. "large market" are the parent's words. And, yes, of course a product that is aimed at a small subset of the market is the opposite of a product that is aimed at a large market. So, it contradicts, as you say.
These are relative terms that speak to the size of the market being addressed, so by definition small vs. large are opposites.
Perhaps the parent meant to say a "small, but profitable market".