It's just capitalism at work. Yesterday's high-quality high-price item becomes today's low-priced low-quality commodity. Quality-focused small brands in every industry and every market need to be constantly improving, developing new product improvements, new niches, new products. (E.g. an even-higher quality version for the luxury niche.)
It's extremely unlikely that Amazon will be be able to raise prices long-term on an own-brand item with "no competition" simply because... well, capitalism. As soon as they do, manufacturers in China or whoever sells the equivalent product at Wal-Mart will list on Amazon for a lower price and make money... so Amazon lowers the price again... and as always, the invisible hand of capitalism keeps prices low for the consumer.