Small co-op insurance schemes can be full of holes. Something that is very important to some people might be left out of a co-op insurance plan, and the subscribers to that plan may have very few options for how to pay for their medical needs. A public insurance option would have national oversight, and be more likely to cover basic preventative care. Having a national standard for care helps make sure that nobody is marginalized by the quirks of a special interest group.
No cooperatives are not an inadequate alternative to the public health insurance option, because cooperatives will offer more choice to consumers. Consumers who choose to join a cooperative can get a better rate, because things that are a result of poor lifestyle choices, like smoking and substance abuse, are not covered. This lowers costs for everyone.