Wait a minute... You mean to seriously say people used to pay a one time fee for software? Next you're going to tell me in this magical Utopia companies allowed you to continue using the same version of their software even after they made updates no one asked for
A lot of them use a phone home/license key model... so it will stop working if you don't pay for the license. Where they don't, ya it's basically a trust system and most big companies would rather just pay the license fee than expose the legal risk of using software illegally.
If we are talking about open source self-hosted specifically, it's mostly consulting, paid support, and offering managed hosting.