Even in the absence of a monopoly, a small number of corporate oligarchs can and will collude to fix prices and wages, without any help from government power. Furthermore, monopolies aren't as unstable as you claim - because they have more capital than upstarts, they can afford to temporarily undersell new competitors, only to raise prices once the threat of competition disappears. This is why conservatives and liberals alike mostly supported anti-trust laws at the turn of the last century.
Collusion in price fixing is not only illegal, but it fails due to someone trying to undercut prices. OPEC always had the second problem.
The US Government spent hundreds of millions of dollars taking on IBM in an anti-trust case. IBM spent more. Nothing came of it.
It was Apple, started in a garage with the benefit of a second mortgage on the home, and later Microsoft, who sold the geeky disk operating system, that took IBM down. The business world is too dynamic for anti-trust to be effective or needed.
I’ll admit that IBM market dominance lasted longer than impatient people wanted. However, it came to an end. This is in contrast with the postal service and their legal monopoly, or government schools which cost the taxpayer way too much for the services they provide.
This is not to say that anti-trust is entirely ineffective. Per Grok:
U.S. Shoe Corporation was a major domestic manufacturer of shoes, and after its breakup, U.S.‑based shoe manufacturing under its brands essentially came to an end.