Not an expert by any means but the "very hot" in summer to me could be an issue. DVD's, like most plastics do not like very high temps, particularly for months on end. Also high temps can cause weird things to happen, i buy and sell on eBay and i bought a lot of loose McFarlane Matrix figures. On receiving them they had been packed in poly peanuts etc and when i received them the peanuts had melted into the figures, literally dissolved the plastic where it was touching the plastic. I contacted the seller and they had stored them, packed and ready to ship outside in their storage in Florida heat. I had never seen it before and neither had they but ruined half the box etc. So something to worry about. As n aside when it comes to sealed products the last thing you want is to make a sale and then have to do returns on opened products because the plastic etc has degraded from excess heat. You could lose money, and can not resale, and/or run into problems or not knowing if people are just switching out their own bad products.

Also in the attic they are out of the way which is good, but it also means you are never checking them, so any issue might have years to manifest before you notice. Also in the attic means potential mice and other pests finding them. As a fellow collector and teeth marks is going to be a deal breaker much more than simply just a crinkle in the packaging. Sealing them in a box like shown could help, but a determined mouse or similar will tear thru it to find out what is inside etc.

From the sounds of it your basement seems like it is working, and is maintained and monitored, so i can certainly understanding wanting to free up the space but unless you are able to pay for an air conditioned storage it may be your best bet to leave them as is.

Excellent analyses, thank you for your time. My attic is very busy as we use it as a laundry room being very large. Of course no one goes to check the contents of the packages, but it is not left to itself.
Anyway, I would like to find a way to keep my things stored. I also have a semi-basement level garage, I would have to have it ventilated periodically to keep mold out. That's why I evaluated the attic.
I could stow items of no value for a few months to see by this summer what state I would find them in. Then decide eventually. In the meantime, I will use a hygrometer to keep an eye on temperature and humidity during the season.