The modern Establishment relies on a mantra of 'There Is No Alternative': potential opposition is guarded against by enforcing disbelief in the idea that there is any other viable way of running society.

my deep attraction to the idea of the 'Overton Window', a concept invented by US conservatives to describe what is deemed politically possible at any given time

[...] Today's Establishment, in my view, is bound together by common economic interests and a shared set of mentalities: in particular a mentality that holds that those at the top deserve ever greater power and wealth. Unaccountable power could become more representative, but it would still be unaccountable. There could be women, working-class and ethnic minority people involved in institutions and systems that threaten democracy, but those institutions and systems would still threaten democracy.

Nor are all police officers 'class warriors' who wish to defend a grotesquely unequal distribution of wealth and power. They do so because they are tasked with enforcing the law, and the law is often rigged in favour of the powerful. [...]

Here, it's worth reiterating that the book is an explicit rejection of the idea that the Establishment represents a conscious, organized conspiracy. Sure, there are undoubtedly specific conspiracies, from police cover-ups to tax avoidance on an industrial scale. Yet the whole premise of the book is that the Establishment is bound by shared economic interests and common mentalities. There is no need for any overarching planned conspiracy against democracy. The Establishment is an organic, dynamic system.