Libertarians should find another term for what they mean by 'free market', because as it stands, they're appropriating an existing term, then telling everyone else that they're using it incorrectly.

In practice the powerful would be able to restrict supply lines and access to market for upstart competitors using anticompetitive contracts, so those not in a cartel would simply wither.

Sure, and in communism there's no need to do even that, because the state provides and we're all happy 20-hours per-week workers.

Every social movement has two sides: a theoretical and a real one. When studying. If we have to accept that China, Cuba and Stalin's Russia what applied Communism looks like, we have to accept for coherence and mainly because it seems to be the truth that what we're seeing (wall street, monopolies, free market not auto-regulating, etc) as applied Liberalism.