Makes perfect sense to me. I don't know how 'canon' it is with Rebels but in the books that created Thrawn for the idea of 'so where were the Grand Admiral's before?', it was written that the rest of them were on that big super star destroyer that the rebels targeted and took out at Endor, crashing it into the death star.
So the idea is Emperor Palpatine is all powerful, so when the super star destroyer and the death star are blown up there's no succession, no one powerful enough to step up and take charge. The rebel's cut off the head but they don't exactly annihilate the imperial forces at Endor, so they all break apart into a bunch of different factions. No single one dangerous enough to directly threaten the burgeoning New Republic, but still with significant firepower that it would be costly and bloody to wipe them all out.
So simply put the threat of Thrawn is not only is he an exceedingly dangerous military mind, but he's the one guy left in the galaxy with enough clout to unite all the Imperial remnants behind him. The king is dead, long live the king!
Well said. I was going to respond with something similar but much simpler wording haha.