It’s also worth noting that Theramore didn’t happen in a vaccum; horde and alliance cities had been getting leveled for some time by that point with the only real difference WRT Thereamore being the speed and efficacy of the assault.
That having been said, Garrosh’s authority was withering in MoP; his caustic attitude and honorless nature caused the other Horde leaders to pull support for him to the point where the Siege of Orgrimmar was effectively a joint operation.
As to Sylvanas: while her actions at Teldrassil were indeed stupid and self destructive, failure to offer any defense to her or a key city in the Horde would have been suicide for the Horde as an organization. And as Zerde unwittingly pointed out: it was met with shock and horror by a sizable chunk of the horde.