Well maybe "taking over the market" is also a monopolistic and anti-competitive practice and if we had say, working regulation that forbade the existence of these behemoth companies then the landscape of value would look a little different.
Monopolies are still bad either way, but I doubt that the failure modes at employee owned monoploes is the same as at outside investor owned monopolies.
There are a multitude of ways that employee ownership can be encouraged, required, or ownership can be limited to employees.
Uplifting the entire global financial system to make a COOP more attractive through regulation wouldn't change this.