I wonder what is going to happen when a Tesla pulls up to a supercharger station and finds all the slots occupied by the " plebs "
Nothing. A Tesla checks whether the supercharger has an available slot to pull into. Worst case is that you have to wait 20 mins or so, but given that Tesla Chargers tend to congregate like sheep (you don't tend to find "just one"), the odds are that someone will be leaving soon.
The average Tesla Supercharger Station has 10 stalls. 10, not two or four with two spaces as is common to other EV charger suppliers. A Tesla driver will be shown how many free spots are in the station that they are looking at on the map, so that they can decide which one to go for.
The biggest problem to opening up Superchargers to the rest of us is that they are not designed for the rest of us, they are designed for Tesla. That makes it very difficult to park and use a Tesla Supercharger because that 3m lead only really reaches to a port where the Tesla port is - or if you have a port on the back or front of the car. For all the EV manufacturers you just stuck with "put it where the petrol flap should be" that's a problem.