Theories

Commonly found

These are the ones I found interesting. Came across on internet, heard from a mouth etc.

Dead Internet Theory

Excessive amounts of dust

Like the one from Interstellar. I feel like it is quite likely.

Solo-self-found deliriums

Inevitable Fungus supremacy

Becoming singular living entity on earth both airborne and on ground.

Imperialism making a comeback

It feels inevitable. At some point, the world will realize that maybe they'd be better off as a part of something greater. Think, how much does a small (not in size, but significance. Or development index) nation contribute to human development and advancement? Do they bring anything worthy to the table? What is the reason for their independence? Or rather, existence?

Sooner or later, citizens of under developed countries, starting with ex-colonial nations; they'll come to think of subjugation not as a taboo, then they'll question if there is any meaning for their independence. Then they'll eventually start voicing these ideas aloud.

You can already see it to some extent; USA's recent ambitions with Greenland, Canada and Panama. Then I think once again the world will start to be polarized between couple of great powers or empires. It is recurring pattern, don't know the exact interval but it happens throughout the history.

Of course this time it won't be as brutal as times of colonialism or imperialism, UN will draft a new framework in order to keep it humane and dignified.

That'll also bring the old problem of governance once more, centralized power/authority always failed in the past, eventually. So if no new system of government is introduced, it is once again likely to fail.