Additionally, there are people that believe that the private sector spurred by competition can provide a better, cheaper product. Personally, I think that's more a feature of new and small companies without entrenched interests and inertia. As you noted the requirements for government contracts have made their entry into bids harder than larger companies that can afford the large upfront bidding cost in time, effort, lobbying, etc.
In my eyes, it's entirely possible the USDS is and ca be successful because it's new. I think it's (unfortunately) entirely possible that in 10-20 years they will suffer from the same bloat and inertia problems large companies and existing government agencies do. I'm not sure how to combat that, but having a way to revitalize large companies or government agencies without losing too much of the existing internal knowledge and expertise would go a long way towards helping combat this, but I'm not sure what that is. There are probably numerous successful companies that have done this that I'm entirely unaware of, so maybe there is a good amount of information on how to combat this...