Everyone who keeps old hardware and media eventually runs out of room in the house, and the loft is where the overflow goes. It is the worst place in the building for the purpose, and the reason has nothing to do with dust or heat.
The loft is a different climate from the rest of the house
Above the insulation, a loft tracks the outside temperature with a lag. It gets colder at night than any room below it and warmer on a summer afternoon than anywhere else in the building, and it does that every day of the year.
For a boxed collection, the temperature swing is the mechanism. Each cycle moves moisture in and out of cardboard, paper sleeves and the labels on disks. Over a few years, sleeves soften, inlays cockle, and anything with adhesive in it, which is most cassette and tape-based media, starts to let go.
A roof leak is slow, local and invisible
The worst case is not the swing. It is a failed roof covering, because a roof is the one part of the building nobody looks at, and a slow leak is quiet.
A single slipped slate or a tired flashing admits very little water, and it admits it in the same place every time it rains. That water lands on whatever is directly beneath. From inside the loft, there is nothing to see until a box is already soft, because the staining is on the top surface where the torch does not reach.
This is why it is worth getting someone up there before the problem arrives rather than after. If you have no idea who covers your area, find a local roofing contractor through a directory rather than through the first ad you are shown, and ask for a condition report rather than a quote. A roof inspected every few years is cheaper than any single collection it is sitting above.
What to look for from the hatch
Go up on a dry day with a torch, after a wet week. Daylight through the covering is obvious and rare. The useful signs are subtler.
Dark fan-shaped staining on the underside of the boards or the felt, concentrated around one point. Rust streaks on nail heads. A tide line on a rafter. Any patch of insulation that is compressed and discoloured, which means it has been wet repeatedly and has lost its loft.
Look along the line where the roof meets a wall or a chimney. Flashings fail before coverings do.
What to do with the collection meanwhile
Nothing stays on the loft floor. Shelving off the joists, lidded plastic crates rather than cardboard, and nothing directly under a valley, a chimney or a soil vent.
Media you genuinely cannot replace does not belong up there at all, whatever the container. The loft is for the duplicates and the shelf-filler boxes. The irreplaceable items want to be in the heated part of the house, where someone would notice a problem in a week rather than in a decade.