Other benefits of trailers:
1. You don't have compatability issues between different car makers and the particular battery swap station or fast charging station that you roll up to. For example a Tesla cannot swap batteries at a better place swap station (meaning these infrastructures are even more likely to go out of business as their compatiblity is limited to a smaller number of vehicles).
2. We know that rapid charging can damage batteries if done too often. A trailer/car combo as I said could be charged separately at a regular rate of charge (perhaps at 240v) overnight, with your 500 mile combined range ready to go in the morning.
3. Tesla could build the trailers in order to provide a security blanket to it's potential customers who would be comforted to know that an extended range trailer would be available for their rare long distance trips. This would drive higher vehicle sales for Tesla and as people come to realize just how rarely they travel more than 180/240/300 miles in one day they will realize that they will very rarely ever need the trailer but probably admit that it was an important psychological crutch to lean on when debating the original purchase of their first EV.
Tesla, once producing cars with a 500 mile range, could phase out the trailers as they will have served their purpose as a security blanket for impractical range anxiety fears. This way nobody, providing expensive battery swapping or fast charging infrastructures, needs to go out of business, as they need never start!