I think the most realistic collapse will be economic contraction.

Caused by whatever, from war, pandemics, climate change, economic collapse, but likely a combination of some or all of the above.

Not mad Max or fallout level, I don't think that's realistic. More like worldwide trade being pulled back to essentials, cities becoming slums of local manufacturing, rural and regional areas becoming largely self reliant minus the few rare but important imports that are just too valuable for people not to risk sailing all over the world for. Fuel, machine parts, solar panels, batteries, ammunition, etc.

Governments still exist, but maintain relvence not through income tax if the grid and the like become so unstable, instead maintaining some authority and relvence through controlling any remaining power grid, and logistics lanes like shipping and rail.

But other capacities severely diminished.

Massive food shortages, limited access to clean water, etc.

I think worst case scenario is more like 1800s levels of local self reliance, combined with modern tech like solar and batteries.

Basically much more like collapses we've seen before in places like Venezuela or post soviet collapse.

Still bad, I think people underestimate how long people can live malnourished and drinking disgusting water. Whole populations around the world today live in slums drinking filthy water and eating things like Pag Pag. And they live long enough to continue reproducing.

So it's still a terrible existence for the majority of people. And even those that are prepared or live rural won't have the access to food we have now. But at least local farms should be able to keep local farmers markets and stuff like that ticking over. Even at reduces capacity. Many likely working in local physical labour jobs if specialised careers are no longer viable.

So still horrible for millions. But people will adapt.