Quite aside from the insanity of the Left, which will end Western civilisation prematurely, modernity will collapse anyway, due to the end of science.
It is scientific revolutions (and technological breakthroughs) which caused and sustained modernity; modernity being the growth of productivity such as to outstrip the growth of populations.
In an important sense it is economic growth (growth in food production and also the production of other essentials) which underpins modernity.
Indeed, as Ernest Gellner saw, modernity depends on economic growth in two ways: firstly to keep people alive, and secondly to maintain coherence - because modern societies are societies of material bribery (rather than societies of material coercion and spiritual bribery, as in the past).
But economic growth depends on significant and frequent breakthroughs in productivity (more stuff for fewer man hours), and breakthroughs in productivity derive from science-technology broadly conceptualised.
And breakthroughs in science-technology - what do they depend on?
Well, I believe they depend upon creative genius - in other words breakthroughs were generated by the high density of creative geniuses (in relatively few cultures) who were positioned such as to make a difference by their work.
Yet there are no creative geniuses any more (or too few, or if there are enough, then they are not able to influence things).
Not enough creative geniuses = no modernity.
No creative geniuses (or not enough of them) means no breakthroughs (or not enough of them); which means stagnant productivity - indeed declining productivity due to the evolution of parasitism (e.g. bureaucracy, economically inactive populations etc).
And stagnant or declining productivity means the end of modernity which means societal collapse leading to population collapse and a new equilibrium at much lower total population (and population density).
The first collapse will be of cohesion, since the population cannot be bribed when there is not enough with which to bribe them.
And to impose cohesion by force and religion instead of bribery would itself end modernity.
The second collapse will be of starvation, disease and predation (mostly by people) - the triple major causes of human mortality
Modernity was therefore merely a temporary escape from the Malthusian Trap, lasting about 8 generations, driven - it seems, by the coincidence of a high density of creative genius in the West (and the conditions which enabled them to be effective in societal transformation) - all of which factors have by now disappeared.
Presumably, society will return to its pre-modern level of complexity - agrarian states, cohering due to a mixture of military coercion and religiousness.
This will happen, sooner or later, but on a timescale of decades not of centuries.
The only major uncertainty is - which religion?