Sorry but you're wrong again. They have neither the room nor the desire to make the room to store that huge assortment of lights. They may indeed keep a small portion of the newer lights for replacement Christmas decorations but the vast majority of the lights are going to the landfill. Christmas light sets are very susceptible to damage from the UV rays of the sun and the wiring harnesses must be disposed of and replaced to avoid fire hazards. Light sets in the greater Orlando, FL area that are not stored in air conditioned facilities are subject to more rapid deterioration.
I agree that WDW is environmentally friendly and they do their part to recycle as much as possible. But the main value of the light sets is in the content of their copper wire and it's just too expensive to extract the wire from the plastic insulation. It's cheaper just to junk the old lights. Sad but true.