How is a fixed price launch contract for SpaceX that different from a cost plus contract for Boeing to build an SLS as far as money going back to the economy? I genuinely don't get how those are meaningfully different.

How does the public domain technology actually matter, other than from an idealistic standpoint? NASA is even spinning off SLS production and management to be more private under EPOC to Deep Space Transport (Boeing+NG). They, along with Aerojet, basically get these sole-sourced, partially because of their non-public IP for making this stuff.