"Man, I've been in their underground part, every 5 years they open for public and open the detectors themselves."

This isn't every 5 years, the detectors are available for the public to visit every winter, and often at other times of the year as well.

"So no humans would need to go down there, if something breaks they could just replace the broken segment(s) and fix the broken one(s) on the surface."

This frankly is not possible. Colliders need constant maintenance. The "Collider in the Sea" is not a real proposal.