> Which means all they have to do is sit on the ability to make product indefinitely and use it as soon as the monopolist attempts to raise prices above cost.

> Which means all they have to do is sit on the ability to make product indefinitely and use it as soon as the monopolist attempts to raise prices above cost. You're underestimating the economic tools at the disposal of large companies. Lucrative contracts with suppliers that prevent selling to competitors, vertical integration where they just buy the suppliers outright, attack advertising, poaching employees, etc. A lot of these can happen even without active malice. Selling below cost is just one of many tools. There are many many ways an economically powerful player can strongarm competitors into submission. None of these are foolproof (and nothing in politics or economics is), but they all make the barrier to entry higher, sometimes almost impossibly high even in the absence of regulation (that is to say thinking of barrier to entry as something that is independent of a monopoly is not a good model, monopolies are intrinsically a barrier to entry).