That's no longer true. A handful of companies have such dominance that the only "competition" is what they allow. They'll either buy or ban any other options. Due to the degree of control those few companies have, all it takes is removing the app from Apple and Google app stores and removing payment processing by Visa and Mastercard.
Technically, competition is allowed. In practice, it isn't. Your view is based on a world before widespread use of the internet, smartphones and social media. A world that no longer exists. Even extremely authoritarian regimes have some trouble maintaining their authority against the authority of the few companies controlling communications and payments and do so by controlling those companies (e.g. China).
People created an alternative to Twitter - Parler. Once it gained enough users to be even a potential alternative option, it was shut down by the handful of companies that have almost total control. Your argument no longer applies to reality. Things have changed.