As a counterpoint: in the 1990s the US government was convinced that Microsoft was an unassailable monopoly dominating all areas of technology. Competition could not defeat it; only government action could defeat it. All of the arguments you made in your post, about size and market domination, were made about Microsoft in the late 1990s.

The government did not break up Microsoft, but then Google came along, and nobody saw it coming. And Apple became a major player after nearly going out of business in the 90s, and Facebook dominated a market category called "social media" that nobody was thinking about in the late 1990s.

It turned out that government action was not necessary, because competition actually did work, and it worked by creating new technologies that nobody expected.

I doubt the people who are concerned about market dominance in 2018 know more than the people who were concerned about it in the late 1990s, and they are probably just as wrong.