It's more than this. It's a federally owned and operated passenger railroad. No other federal agency is expected to pay its own way. Why should Amtrak?
"Motorists pay the full cost of federal highway funding."
Not true. Not even close. Locomotives use diesel fuel. Billions of gallons of it. A few years back, I read that only the US Navy used more diesel fuel than Burlington Northern. These billions of gallons each have a tax on it. Tax $$ that goes to federal highway funding. Fedreal highways used by trucks. What other industry is forced to subsidize it's competition?
Not everyone wants to get from point A to point B the quickest way possible. Passenger rail service is still the only civilized way to travel.
And, it could be so much better and more viable with just a some good business sense, imagination and vision. And that does NOT mean cutting existing routes, especially the long distance western routes.