Theory: The Heralds are responsible for Honor's fate.

[potato]

This one is pretty straight forward:

Honor invested a lot of his power in the Heralds. This power should have been returned at the end of the last Desolation but wasn't, due to the Heralds decision to forgo damnation. This means that Honor was still highly invested on Roshar, while Odium was not (no voidbringers, no investment).

Odium took advantage of this discrepancy of power, splintering Honor. Now humanity relies on The Stormfather (a shadow of Honors power) and the Heralds (even lesser shadows of their former selves) for salvation. Odium will win unless they can somehow force him to invest a great deal of his power in a champion. Even then, they'll probably need some help from Cultivation to succeed.

Less likely addendum theory:

The Heralds were the leaders of the Silver Kingdoms and completely lost it when Honor was splintered. This led to the locking of the Oathgates, the destruction of Stormseat, and (indirectly, through the destruction of Stormseat and other wars where each Herald tried to keep any of the others from acquiring enough power to Ascend to become Honor {despite the fact that none wanted the requisite changes or responsibilty}), the Recreance.

[RShara she/her]

WoB is that they're tangentially related, but not directly responsible.

 

Q:  Was Odium able to splinter Honor because the Heralds abandoned the Oathpact?
A:  Good question. Um, their abandonment of the Oathpact is related...but mostly tangentially.  If I was pinned down on that, I would say no.

[potato]

Thanks! You always have a quote handy to crush a flawed theory (this is where I'd insert a smiley if I was given to that sort of behavior).

[RShara she/her]

Sorry! 

[kaellok he/him]

On the bright side, it's a really good theory!  And not entirely wrong--just, like, 95%.  (I was right there with you, too, thinking that it had to be the Heralds breaking the Oathpact.  It makes ALL OF THE SENSE, especially when we find out for sure that KR breaking their oaths kill their spren.  That whole bond thing is somewhat related to what Honor did with the Heralds, so it only seems logical--but, alas, no.  It makes me sad, because it just straight up killed my pet theory of Honor reforming after the Heralds re-swore the Oathpact and started living it again.  But, since that's how I would have done it, and Sanderson is a much better storyteller than me, I look forward to finding out what all is going on.)