Based on my own non-expert personal musings on the subject, I would say that natural monopolies can and do exist in capitalism. It's just that under normal circumstances they would be difficult to sustain over a certain period of time based on several factors. Without some mechanic (often political) in place to prevent a new form of competition or technological progression from developing in the market, then a natural monopoly would likely be reduced by natural market forces.

If you look throughout just American history you can various examples of this. Big, strong companies that eventually fade away to disappear because of failure, lack of vision, or whatever other decisions that held the company back.

It's just that there is very few examples of a true free market, except for maybe on a local level, so that we'll likely never know for sure.