There will always be a big loss for the new boat owner, custom or otherwise. Your boat has held it's value well, but I think it's design is the primary reason. I know of a few tris that aren't worth a plugged nickel, just like so many monohulls today. And those are a mixture of mostly glass but some wood, metal. Good wood designs have lasted, so have good glass designs.

Unpopular glass designs are going to the landfill, bad wood is going to compost.