And that's precisely why monopolies rarely form (not impossible, but really difficult) in commodity markets.
Anyway, it's not as important as when the monopoly is established. Any competition would have to bootstrap itself. Until competition is reestablished, market distortions ensue. They can be as large as the barriers that prevent competitors from entering the market.
In a market with strong network effects (e.g., operating systems), you are correct. It costs money to switch from windows to linux, and this may preclude a switch for a small cost gain.
In a market for commodity goods, this is not true. It costs nothing to switch from Alcoa aluminum to little guy aluminum.