It seems to me that the narrower the value proposition is, the harder it is to build a minimally viable product. Not sure if the following holds up everywhere, but try this broad-to-narrow analogy:

• [broad] If you want to provide reliable ground transportation, you could build a locomotive.

• [less broad] If you want to provide high-speed, ground transportation, you need to build a bullet train.

• [narrow] If you want to provide comfortable, high-speed, ground transportation, you need to build a bullet train with leather seats and beds.

• [super narrow] If you want to provide non-stop, nationwide, comfortable, high-speed, ground transportation, you need to build a bullet train with leather seats, beds, and a nuclear reactor.  

A narrow value proposition does not equate to a narrow market opportunity, it's just harder to build.