usually the product vision comes from the domain expert. any one or more could be domain experts.
the vision for a core technology startup often comes from the hacker.
the vision for a lifestyle / design innovation startup often comes from the hipster.
the vision for a consumer / b2b business model innovation often comes from the hustler.
take a look at the founder of
clerky.com, for instance:
https://angel.co/darbywong has both product development tech skills (CS) and domain expertise (JD + startup work experience). he's the domain expert and the hacker, which is the combination Y Combinator selects for. presumably the other founders on the team have hustle and design.
this model assumes there is an innovation of some kind present.
sometimes none of the three roles has sufficient domain expertise to articulate any kind of innovation insight.
in those cases it's usually up to the hustler to out-execute the competition in a crowded space which often includes Rocket Internet.