More practically, history has shown us that the GPL is necessary in order to ensure that companies don't short the community, suddenly improve everything and leave us with 99% of people using the proprietary fork. To illustrate my point, the clearest example (and the only large one) is the free distributions of BSD, mainly used by companies developing proprietary forks for it and contributing back very little. On the other side is Linux, used by companies for its freedom, security and reliability. The companies using it contribute back because they have to, continually making it stronger and always remaining free (unlike BSD, which can have many proprietary and untrusted libraries which cannot be used with security or reliability in mind). Linux is therefore getting larger only thanks to the GPL and its strong enforcement.
And this example can be applied to the entire copyleft vs permissive license debate, showing how software being free benefits the people more than people being free.