The estimated total energy Sun push toward Earth have little to do with the potential fraction of p.v. we can really deploy. There are enormous constraints from water, too hot places, areas subject to floods, wildfires, ... without counting the real capacity to build and replace solar panels and batteries witch so far no one really know.

About projects, in any field, big, complex projects normally never get done well, smaller normally succeed. That's is. Since it's valid from IT to A&D why it should be different for something else?