Spoilers I believe I have a VERY good Theory for the Burn...

[Deks]

Not the mirror universe, but perhaps "stealing" it (or, rather, mining it) from other universes, mostly uninhabited or little inhabited ones. That seems reasonable, even if the result is nonsensical.

I objected mainly to the term 'stealing' and 'UFP' in one sentence.
The Federation didn't strike me as such, and if they tried mining dilithium from other universes and that caused the Burn, I would think it would have been pretty obvious as a likely causal factor by the time the even occurred.

They already had the ability to distinguish with their scanners different quantum resonance frequencies of individual universes even as far back in the 23rd century... I think finding a dilithium crystal that exists across dimensions would have been pretty obvious/evident to UFP capable of hopping universes easily.

 

[Dutch K.]

My amateur theory is the Terran empire defeats Starfleet after Discovery left.
Becomes that which they destroyed(Starfleet), as most viable option to remain, in power,and truth of its motivation and goals hidden from the view.
Which is the return of Georgiou(?), as Empress of Terran empire in future.
Mainly because of the knowledge she might possess of secret dilithium supplies she created that are still hidden in the future.
Might be that a flaw or problem with future dilithium that doesn't exist in older dilithium or separate dimension dilithium. Could be a reasonable option also to bring Georgiou to future as a
savior/ failsafe against destruction of Starfleet/Terran empire.