I don't have a "startup" but I run a business that utilizes freelancers. The idea that freelancers can't work in an agile manner and respond quickly is nonsense. Several of my best freelancers are on retainer. I lock in availability and they love the guaranteed income.

Sure, I don't have 40 hours a week from each of them but there's nothing magical about a 40 hour work week. Agile sprints are time bound and time bound doesn't mean "40 hours".

By the way, most startups don't have an idealized feedback loop where market feedback is coming in at a rapid pace on a constant basis and your developers are busy 5 days a week responding to that feedback.

Most startups take longer to get going than the founders would like. If you're paying a salary to your technical co-founder a lot of it is likely going to go to waste because you probably won't be able to utilize them 100%. And even if they're not taking a salary they're going to get bored (and might jump ship) if they're expecting things to take off overnight.