What you're saying is all true, but the Superchargers are and always have been partially about marketing.
If you go hang out on ICE forums, you'll see people talking about how BEVs are a "leash" that ties you to your home. We all know that's not true, yet it's a common belief among the general public. The Supercharger network serves two purposes: 1) making it possible in practice to drive your (Tesla) BEV everywhere you want to go; and equally-importantly 2) challenging the common misconception that you can't drive your BEV everywhere you want to go.
Is a transcontinental supercharger route kinda stupid at this point? Well, yea, from a practical point of view it is. It helps very few owners make trips they want to make. Is the particular transcontinental route they chose kinda dumb? Well, yea, I-90 adds hundreds of miles to most transcontinental trips.
But the transcontinental supercharger route isn't (at this point) really meant for practical trips (though it does help build that capability for the future). It's meant to burn into the public's consciousness that "Wow, I could drive a Tesla from New York to Los Angeles in about the same amount of time as my current car. I could totally see myself doing that." Most people simply don't have enough vacation time to make that kind of drive a practical proposition, yet virtually all car buyers judge cars on that criterion. Consequently, it's important for Tesla gallery employees to be able to tell prospective Tesla buyers that yes, you really can drive this car from NY to LA.