There could be more eyeball-tentacles flying toward Azeroth as we speak. The first "batch" of Old Gods were sent out Lord-knows-how-long ago (it wasn't given a date in Chronicle, just a time in prehistory), long before any mortal races now existed. Others wound up on random planets. The reason the Void Lords flung out Old Gods randomly was because they didn't know which worlds had World-Souls.
It's unknown if they can actually accurately send a new Old God through space directly to Azeroth now. They certainly didn't get every planet anyway, since we never heard of/saw any Old Gods on Argus, and Outland's only Old God was
summoned by Arakkoa in Burning Crusade.
The problem with killing an Old God was that the they had burrowed deep into Azeroth, so she would be damaged if the Titans pulled them out - that's how the Well of Eternity was created. On top of that, "killing" them doesn't really do much good. Y'Shaarj was still kicking in the form of his heart, and his heads turned into the seven Sha, with tons of lesser Sha also coming from him. They also studied the Old Gods in order to figure out how to take care of them for good, and accidentally created G'huun. The Old Gods are said to be "outside the cycle" and we've seen "dead" Old Gods
whisper to their victims and
summon minions, so destroying their bodies may not do much of anything in the end, if they simply go back to the Void, or they exist in Ny'alotha.
Not to mention, the titans built a device to reoriginate the entire planet while leaving the World-Soul unharmed, so it's not like the Titans didn't want to kill the Old Gods themselves, they simply didn't want to kill everything else with them while hurting the planet.