The superchargers are a shared resource.
Tesla/EV drivers must adapt to effectively sharing the chargers.
For-pay public chargers usually charge per minute connected, not for charging time or energy consumed - if a vehicle stays connected after charging is completed, charges will continue to be incurred until the vehicle is disconnected. This usually discourages drivers from leaving their vehicles parked for extended periods after charging has been completed.
Tesla probably should have implemented terms and conditions for supercharger use when they installed the first superchargers - defining the practice Tesla drivers should follow in order to get access to the superchargers - including disconnecting after charging is completed and not parking in charging spots (and not charging). And, at some point, it's likely Tesla will be forced to require all users of the supercharger network to agree to T&C's before being able to use a supercharger.
Even for owners getting FUSC (Free Unlimited Supercharging), Tesla drivers should get used to disconnecting shortly after charging has been completed, even if there are open supercharging spaces.
We shouldn't expect Tesla to overbuild the supercharger network because owners (even those paying for charging) aren't willing to take reasonable steps to share the superchargers.