I think Amazon will support crypto of some kind in 2018. Only makes sense. They want to make it easy as possible to buy from them. They see huge increase in crypto market cap. Also would not be surprised if they launch their own coin. Bezos wants it all. 
There is no actual relation between increase in the crypto currencies market capitalism and amazon accepting it because of that. Popularity of the crypto currency is another part, investors making investment into it is also another part. Amazon is online shopping centre which is meant for easiness of the customers. Most of the people don't know what crypto currency are and how they really work. I mean how many people earn the bitcoin online? And then they go for shopping? I think the number is not much, so most of the people will need to buy the crypto currencies before they can go for shopping with crypto currency. This will make the whole process very difficult. I mean the fees and transactions are not making sense currently. So its still far from accepting the crypto currencies really.