Here's an example that I've actually done twice in the last two years: Denver, CO to Austin, TX. That's about a 900 mile trip, and a long but tolerable day -- from Denver south to Raton, NM and then southeast through Amarillo, south to Lubbock, and then southeast again. To stay on the supercharger network I'd have to go all the way over to Oklahoma City (on either I-40 or I-70 to I-35), adding 150+ miles to the trip, not to mention longer and more frequent stops that would likely necessitate adding an extra half-day of travel time each way.
My brother lives in Chicago. His trips to Indianapolis would be covered, but not his trips to Denver.
Dallas-Fort Worth is a huge population center, with great supercharger coverage leaving north or south, but not east or west. If you're heading to Atlanta, Memphis, or St. Louis the supercharger network is going to be really inconvenient.
And while the majority of the US population does live in major cities connected by interstates, the majority of the US population has at least one close friend or family member they'd like to visit, who isn't connected by one of the supercharger-served routes. If you made graphs not just of population centers, but of close personal connections, your "graph of graphs" would have quite a few connections that aren't covered by the network. (While truck traffic and passenger traffic are not identical, figure 2.8 of https://www.fhwa.dot.gov/policy/otps/bottlenecks/chap2.cfm suggests quite a few heavily-trafficked routes that aren't covered by the supercharger network.)
It's not my intent to be overly negative. I'm just being realistic, presenting the analysis I did myself about 6 months ago after my old car got wrecked in a hit-and-run. The majority of the time, I'd be happy with a Tesla and the supercharger network, but a couple times a year it would be a massive inconvenience. This is an area where the difference between 95% and 100% is huge. Tesla/superchargers are getting there, but it'll be several years before the infrastructure is in place for a pure electric plugin with a 200 mile range becomes a truly viable vehicle for the majority of people.