> Free markets have a well established, well understood "tendency towards monopoly."
There is actually a lot of debate about what causes this.
The argument on one side is, of course companies unconstrained by laws would collude with each other to fix prices etc.
The argument on the other side is, as soon as they start doing this it becomes profitable for someone else to enter the market. Is the cartel charging $40 for something that should cost $20? Go around to all the customers and tell them you'll sell to them for $25 tomorrow if they agree to a long-term contract today to buy from you at that price. Then you have enough guaranteed customers to pay off your new factory, so you can build it without being bankrupted by the incumbents slashing prices. And because anyone can do this, the incumbents don't collude to charge $40 to begin with because they know they would invite it.
Under this theory, monopolies form because of regulatory capture. The large incumbents don't want to compete in a commodity market, so they pay off the government to erect barriers to entry. Then once no one new can enter the market, they can buy each other or collude or use conscious parallelism without risk of new competitors.
It seems to me that it's kind of both. Without the second one, companies would rarely if ever get big enough that no one could compete with them. But once they are, somebody's got to break them up.
Most importantly, we need to fix the regulatory environment so that it facilitates rather than impairs competition, or it will just happen again.