Arguably even that’s not the whole case. Technically, Azeroth’s the last known World Soul because she was the last one the titans found, and she’s only the most powerful known World Soul because she’s the most powerful they ever found.
Yet even Chronicle indicated that the titans only ever managed to explore a tiny fraction of the totality of the Great Dark, so in theory, there could be any number of dormant, undiscovered World Souls “out there” that just haven’t yet matured to the point of attracting the scrutiny of the Pantheon or the Burning Legion, since both were arguably being drawn to planets by what were effectively the possible symptoms of possessing growing World Souls (and by an astronomical margin, even with those symptoms, most of the time they still didn’t.)
Heck, even the Legion’s search would have always been somewhat stymied by the fact that in order to invade a given planet, they have to have some way of actually knowing it’s there, be it contacting the native life in order to arrange to be summoned or at the very least, noticing some event upon it from within their own domain. Otherwise beyond Sargeras presumably knowing the locations of Reordered planets the titans had already visited, there’s basically no solid point of reference for them to know where a given world is from within the Twisting Nether if there’s nothing on its surface yet that’s developed enough to “announce” its presence (like, say, night elves’ use of the Well of Eternity painting a target on Azeroth or the Genedar’s crash-landing on Draenor sending out a huge wave of Light energies.)
So in theory, there could be any number of worlds in the Great Dark that just haven’t yet developed sufficiently to draw the gaze of these cosmic agencies, and from among those, some tiny number could perhaps turn out to contain World Souls of their own. Perhaps even one or more that would eventually grow to eclipse Azeroth.
And since he could never be sure that wasn’t the case, Sargeras’ Crusade could never end. Because while Azeroth being so powerful is relevant, what’s arguably even more relevant to him is that the Void Lords don’t even need the most powerful World Soul to accomplish their goal. They only need any World Soul to create an unstoppable Void Titan, so he had to burn everything because with countless Old Gods cast across the cosmos, always potentially crashing down onto even more worlds at any given moment, even with his infinite army of demons time was always against him and he couldn’t take any chances. As long as anything still existed at all, he’d have to continue scouring creation for eternity, forever looking for any worlds that might escape his notice and might harbor even the slightest statistical possibility of housing a World Soul.
Sargeras set out to destroy all life, despite it arguably being impossible for him to ever reach a point at which he could know he had succeeded.