Seems like a bad long term investment, since there's a plan for them to be worthless eventually. Economically speaking, if the market is rational, the price should tend down over time.
Of course the market may not be rational (it's obviously not super liquid, either), and it's very plausible the price creeps up over time before eventually crashing, or that we never get to widespread IPv6 adoption after all. Maybe you have some insight that they are underpriced at the moment and IPv6 adoption is further away than the market thinks. But I wouldn't contemplate this as an investment unless I had some plan to collect rent for the assets to make up for the expected eventual depreciation.
Of course the market may not be rational (it's obviously not super liquid, either), and it's very plausible the price creeps up over time before eventually crashing, or that we never get to widespread IPv6 adoption after all. Maybe you have some insight that they are underpriced at the moment and IPv6 adoption is further away than the market thinks. But I wouldn't contemplate this as an investment unless I had some plan to collect rent for the assets to make up for the expected eventual depreciation.