We're not talking about regular guards though. We're talking about the officers.
The people who the World Government deems powerful enough to stand between the most powerful criminals on the planet and the rest of the world. Magellan seems qualified, and Shiryu probably was as well. But nobody else seems qualified.
Which, I guess, is why this is happening now. The WG put underpowered people in charge of keeping the most dangerous place in the world sealed up tight, and now all these villains are going to get free because of it.
End of the day, the staff of Impel Down wouldn't even be able to contain two Level 6 prisoners escaping simultaneously. Because Magellan would only be able to fight one while the other would end up soloing the entire rest of the facility.
Perhaps it wouldn't be as much of an issue if it WERE a refreshing change of pace. Unfortunately, the Impel Down arc directly follows the Amazon Lilly arc, where Luffy ALSO completely outclassed every threat thrown at him.