I do have to smile at this thread. If it was excess for a person along the way, isn't it likely to be excess for most future pilgrims? There can't be many pilgrims looking to pick up hairdryers or subzero bags or tents along the way if they didn't bring them in the first place.
Obviously, some things can legitimately be recycled with a reasonably high likelihood of reuse by future pilgrims. Other things will never have sufficient demand to consume the ever growing supply. At some point, I can only imagine that they get thrown out. I suspect walking sticks and pocket knives and sleeping pads top the list.
But I would tend to think that a high percentage is more applicable to redistribute to those Spanish citizens in need of aid due to poverty or unemployment (clothing, boots, sleeping bags, liter-sized shampoo and conditioner, etc.). I envision something like Goodwill or ARC in the US; although, they too get a lot of stuff that isn't worth reusing and then they have to throw it out. As such, these charities become dumping grounds that make the donor feel good, even though the donation was pointless.
Thus, I would think that, as we consider what to leave, we ought to ask two questions --- is it in good enough condition to be reused by another, and is it likely to be of use by a future pilgrim or someone in genuine need? If not, then we simply need to bin it ourselves (bad condition) or take it back home with us (good condition). Otherwise, we risk simply making the Camino a dumping ground, albeit better organized.