But NONE of the Hacker News commenters addressed Sony and 
  Panasonic’s continued investment in high-density optical 
  disc technology. [...] There has to be a business reason 
  for the continued investment, i.e. customers prepared to 
  buy a lot of product in the future and buying a lot right 
  now.

If Amazon was the only customer for high-density optical storage, they'd be crazy to invest in developing it, because Amazon will have all the negotiating power in the relationship.

There must be other customers somewhere if two companies are continuing to develop this stuff.