We are demonstrably not free to reimplement all closed applications. Slack and Discord are two notorious offenders who consider third-party clients a TOS violation. We can argue over the technical feasibility of enforcing this policy, but plenty of people are forced to use these applications as a condition of their employment and are unable to replace them with native counterparts.

We can also argue that employment and computing in general are voluntary activities, simultaneously preserving and significantly weakening your argument.