[fyi, "natural" monopoly is the kind of monopoly where competition isn't efficient or desirable: roads, power, sanitation, network protocol, etc. it's not some kind of justification of unregulated consolidation]
A libertarian might hold this, but not one which had any experience in economic ideas, especially the microeconomics used in market analysis. At least, not without occasional bits of brains squirting out their ears. The problem with unregulated monopolies is that any competition in the market will lower prices, raise production, and drop profit down to a point below where economic resources can be sucked out of the economy as rents to the monopoly owner. Where competition exists, monopolies can't compete except in the short term to drive the competition out through dumping, coercion, bribes, kickbacks, etc. It's not just a definitional problem: unregulated monopolies are badly matched to to their markets. Someone worried about resource depletion (as some libertarians are) might justify monopoly as a way for the owner-class to starve the population back to sustainability as well as fund (through rents) the kind of security theater necessary to keep the population from successfully revolting. I don't think that's been put forward as an explicit proposal by mainstream libertarians like the Kochs, but it seems to be the direction some of the collapse-bothers are going.
It's certainly the underlying scenario of some dystopic fantasies I've seen. Snowpiercer is an excellent example.
Without going quite that far, most libertarians slip over to a scapegoating compromise: it's government which makes monopoly possible. For capital-intensive natural monopolies (utilities, roads, transit) they're not entirely wrong. It's just enough right to allow them to conflate natural monopolies with criminal monopolies. That's the wiggleroom they need to make political choices (and talk a lot of nonsense) outside of any responsibility for consequences.
Given the subversion and anti-democratic violence needed to dismantle democratic-republican institutions, that wiggleroom will always be there, just as it was for communists who could always point to the fact that full communism was never really given a chance to work on a large scale. So the idea of laissez-faire will never go away precisely because it is incompatible with a real world which becomes more and more democratic-republican.