In an early stage startup, it makes sense that one of the founders will run the product side of the company. But really good designers often want to have a good amount of control over the product, knowing that putting a glossy coating on a product with bad UX is a losing proposition.
This often means getting more involved in product research/strategy/prioritization process. This can be problematic, since non-founding designers are often hired mostly for their UI design/front-end implementation skills.
My feeling is that very good designers are ideally suited to taking a larger role in running the product side of the company. If startups are willing to offer this, I think they can lure more good designers to their company.
Personally, this is the position I'm in. I'm an experienced designer, looking for an early stage startup to work with. Many founders I speak with are hesitant when I say that I'm interested in working on the big picture vision for the product. I'm still looking for the right fit in this regard.