They'd have to completely undermine the world government to turn it into a dictatorship, and get the countries underneath it to go along. Is that much easier than getting power over the entire world the old-fashioned way?

I mean, sure, if you want a toothless world government with no power that wouldn't be able to keep countries in line. In which case I'm happy to report we already have this: its called the UN.

For anything else you would call an actual government, the highest body has the power and physical force to keep those under them inline. Even in the USA, where states rights are a thing, the federal government could easily force them to do basically whatever they want.

Huge countries like India, the US, and Russia have come under the thrall of men whose position in the feature space is near words like “deranged” and “terrifying”... there’s no reason to believe that the same thing could not happen to a world government

Imagine if there's a revolution, but it's world population - 1 this time.

Fully fledged totalitarianism isn't overthrown by revolution, a worldwide totalitarianism isn't some kind of necessary stage before universal peace -- it would rather make peaceful, dignified relations of people to themselves and another completely impossible, and maybe for good.

> At this point we find ourselves confronted by a very disquieting question: Do we really wish to act upon our knowledge? Does a majority of the population think it worth while to take a good deal of trouble, in order to halt and, if possible, reverse the current drift toward totalitarian control of everything? In the United States and America is the prophetic image of the rest of the urban-industrial world as it will be a few years from now -- recent public opinion polls have revealed that an actual majority of young people in their teens, the voters of tomorrow, have no faith in democratic institutions, see no objection to the censor­ship of unpopular ideas, do not believe that govern­ment of the people by the people is possible and would be perfectly content, if they can continue to live in the style to which the boom has accustomed them, to be ruled, from above, by an oligarchy of assorted experts.

-- https://www.huxley.net/bnw-revisited/index.html

It's not that we now have less risk for the things that were described by thinkers in the 20th century, it's that we became more complacent and cowardly, less ambitious and more comfortable, and are rationalizing it. We don't know more, we just are less able. We don't rise to the occasion, we pull it down into the gutter with us. This is the whimper our betters have seen coming.