The one thing that startups need is freedom to make what people want so that those people will freely choose to pay the startup (how many startups existed in the USSR in 1970?). OSS is just a nice-to-have once you look at it that way. If it was a necessity, you'd have to explain how there were startups before anything was open source.
Edit: OSS would still exist in some form even if there were no government funding for it. You know how some people say they do it just for fun? I think that's true.