I recommend you actually read up on natural monopolies. It's a well understood and universally accepted phenomenon (at least academically; obviously there are vested interests who prefer to deny their existence despite clear facts).
Every time people show me a "natural" monopoly I find regulations around it that corrupt the market.
People believing in the so-called natural monopolies lack fate in the entrepreneurial drive, creativity and innovation of the free individuals working hard in their own interest, for their own betterment.
Capital costs do not create monopolies, just an obstacle solved by raising capital. Government regulations create monopolies - there is no way to fix those.