Because if we wanted to start thinking of that possibility, there are a few scraps from her time in Rhuidean that would suggest she might have something to do with this.

I mean, the Seanchan, as they are now, are dead set against channelers and want them leashed. We've seen, from Avi's trip down future-memory lane, that this will not change (assuming the future is alterable) unless she does something to change it, as she's the only one who knows this future.

We also know that future ancestors of the Dragon recalled 'the Empress around the time of the Last Battle' as 'reasonable', hinting that she would have been more likely than anyone in the Seanchan's future to not be hell-bent on domination, and by extension, leashing all channelers.

So, that could only mean, if this interpretation is correct, that Avi does something in the last book to either save Tuon's life so that she can be convinced to 'right the ancient wrong', or just plain convince her. I mean, once we get into the chain of causality, I suppose she could feasibly save Mat, who could convince Tuon, etc, but anything beyond first-order change at the hands of Aviendha is really a bit convoluted.

Still don't know why the hell the book was called 'Towers of Midnight' though. A cool name? Sanderson thought he'd get to that point in the plot but changed it after the title had been announced? I dunno.