How Anet can provide a plausible explanation as to why Primordious has not caused a chain of volcanic incidences or seismic instabilities crippling all life on Tyria. Within the game there is considerable proof that Destroyers are capable of agitating volcanoes. This is shown in mount maelstrom and in the ring of fire islands.
Now we must ask the question why has there not been any significant seismic activity or volcanic activity in Tyria over 250 years. It must be due to one of the following reasons: Destroyers have not figured out how to purposefully cause earthquakes. Primordious does not want to cause a mass extinction event. Dwarves are still keeping the Destroyers at bay. Primordious does not understand causing volcanic activity would annihilate Tyria. I believe this list covers the most believable reasons as to why Tyria isn’t covered in ash.
Time to examine the plausibility of each scenario, there is little evidence to support Destroyers are capable of causing significant seismic events. For example, Mordremoth somehow tunnelled half way across Tyria with vines and never caused any seismic events. Various tunnelling species such as dredge Wurms do not cause seismic events. It is safe to say creating vast tunnel networks in subterranean Tyria does not cause seismic events for some reason. It does not matter whether it is true in real world science or not but the fact that there is consistency in the GW2 universe is enough. Therefore I conclude destroys currently are unable to cause seismic activity in any meaningful way.
Primordious may not want to cause a mass extinction event. This could be due to various reasons one of which is causing a war between Elder Dragons. It has been shown that Elder Dragons have no desire to cause direct conflict between each other. Another reason could be Primordious living things are in some way important to the Elder Dragons diet thus should not be completely wiped out.
Destroyers could be actively attempting a Doomsday plan but are unable to due to dwarven intervention. This theory seems increasingly less plausible. Significant time has passed with dwarves believed to be fighting the Destroyers without other races help. There is no in game evidence to my knowledge showing any of the known races currently aiding the dwarves. Ogden is in the surface helping the Orders. Our second encounter with a dwarf is Rhoban whom is in broken state. He informs us that all his friends Dwarven buddies which were three are dead and the devices designed o stop the Volcano Islands from erupting could not be activated for this reason. Therefore it must be that there are not many dwarves around to fight the destroyers as compared to the past or that more recently the Dwarves can no longer hold against the destroyers.
Primordious has no idea that agitating the Ring of Fire volcanoes and the Shiverpeak volcanoes could cause a mass extinction event. Destroyers have been shown to have goals and to act in an intelligent manner to achieve their goals especially when guided by a champion. When closely examine the destroyer intelligence is very linear and goal oriented. They are capable of organizing themselves and performing coordinated attack but lack the ability to think critically. Zhaitan for example was able to raise legendary historic figures to command and think critically for him. Primordious due to his selective corruption of the inanimate and things or things lacking the ability for higher thought impairs the Destroyer’s ability to enact any complex or scheme. I find this reason the most plausible and thus dub Primordious the dunce of Elder Dragons
TLDR: Primordious is the dunce of Elder Dragons he cannot think critically.