It comes down to task bandwidth and specialists are often dead weight and bottlenecks except when they're not, but that's not often. Stay diverse. Stay dangerous.
Divisions and collaborative ventures that start in a large companies' research lab and eventually split off or are sold to another party are not startups.
A stable system must have high diversity and high connectivity. If you do not have a network with strong connections to many people of many backgrounds, you will have trouble surviving/understanding changes in the world around you and you will eventually become irrelevant. The startup environment is incredibly taxing on a person and the group. If the people in a startup are not well connected and diverse in knowledge, they will not be resilient enough to survive internal or external conflict. In small groups, it only takes one person with a small network or a limited scope to make it all crumble when the pressure is on.