The cost of providing and OS and an entire ecosystem of device drivers and an an entire ecosystem of apps is very high.
But even if you manage to build that, the cost of training a significant percent of the population into using and developing for your platform is much higher. Getting a significant presence in the "brain space" of a population is very expensive. Having people switch to an alternative is even more expensive due to, among others, human network effects.
Driving adoption is a matter for all products in the market, not specific to operating systems. You can make it easier with familiar UI, advertising and bundling your OS with hardware (although that may be anti-competitive practice it seems to be widely accepted where I live).
It's a matter of degree. To simplify, the costs of switching are linear in the complexity of using the product. I apologize that I don't know how to precisely model human network effects (you need to learn the product from someone) on top of it, but intuitively the societal costs are super-linear.
Computer systems are by far the most complex products human kind has ever produced. Given the simplistic model sketch above, the costs of switching in the computer industry are the highest humanity has ever seen, likely by orders of magnitude. Fun times.
Generally, high complexity of the implementation does not necessarily translate into high complexity of the interface.
Could you explain how an operating system is a natural monopoly?
Development of a new OS isn't cheap, but isn't prohibitively expensive either. It also isn't undesirable AFAICT.