The Decreasing Viability of YouTube as a Platform for Independent Creators

Dave Cullen makes an interesting point to combat the notion that Google and Twitter can censor whatever they wish because they are a private company, with his argument that they are essentially a monopoly that built their base on the foundation of the First Amendment. Basically, they used the First Amendment to become a monopoly, and now that they have absolute control of their broad niche, are silencing voices that they disagree with. What are your thoughts on this argument?