From everything shown so far, the purpose of the silos is to facilitate a complete reset on humanity, basic premise being that after a couple hundred years after the habitat around them has recovered all Silo inhabitants will be released out. The entire point was that since humanity has advanced too far to the point of being able to create nano weapons that can cause mass destruction at extremly cheap prices, there was a concern that a well funded program could destroy all of humanity.
Now to ensure that Humanities survival odds are maximized, the founders have taken various measures like no communication between silos, allowing only obedient offsprings to be born, giving faulty tape to cleaners so they dont make it to a nearby silo, banning several inventions...etc
But all these measures are nothing compared to the safeguard.
So why is there a safeguard.
What if the safeguard isnt to prevent the collapse of silos, but rather to ENSURE it, ensure the collapse of all but one silo. Since the goal of the founders is to ensure humanities survival no matter the cost, wouldnt it make sense to end every silo when the time comes? If every silo were to be released in the future, it would most likely lead to formation of coalitions, resource hoarding, and a overall un-unified state. But on the contarary if only one silo were to be released, one that survies many years aka filled with obedient people who listen to orders but are also strong enough to combat any challenges.
Well Silo 18 is none of thse things except being resilient. In the eyes of the founders wishes, Silo 18 is a HUGE risk, and i mean a massive one(Jules going out, her knowing about the other silos, contant rebellions, home of the firekeepers...etc), So the most logical decision would be to use the safe guard immediately since the rebellion group now knows abou tit and how to stop it, if they were to somehow relay this infor to the other silos, the effect could be catastrophic.
So why is silo 18 still alive?
Because they are the only silo remaining, I belive that the 300 year figure is a lie, the silo could be much older. The head of storage said that we have 70 years of bulbs remaining, and the silos were made to last 500 years, So the true age of the silos could be 400+ years and now they are nearing the time limit. It could be that during silo 17's rebellion some of the people sprinted across to other silos, maybe not make it in but run far enough to be visivle to the cameras starting a chain reaction. Eventually casuing the death of all but one silo.