IPv4 isn't an "investment" for Amazon. They didn't buy them up with the hope that the price will increase and they'll make a profit. They buy more IPv4 addresses because they need those addresses to continue serving customers. They will likely never, ever sell those IPv4 addresses.
And what's more, they will need to continue to buy more IPv4 addresses as they grow and their customers needs for address space grow. Amazon actually has an incentive for IPv4 to become less expensive so that further purchases of addresses are less costly to their business. And the best two ways to do that are: 1) encourage adoption to IPv6 so that IPv4 becomes less expensive and 2) encourage adoption of IPv6 so that Amazon needs to buy less IPv4 addresses.
And what's more, they will need to continue to buy more IPv4 addresses as they grow and their customers needs for address space grow. Amazon actually has an incentive for IPv4 to become less expensive so that further purchases of addresses are less costly to their business. And the best two ways to do that are: 1) encourage adoption to IPv6 so that IPv4 becomes less expensive and 2) encourage adoption of IPv6 so that Amazon needs to buy less IPv4 addresses.