I've got several use cases where my customers are now insisting that the product support IPv6. (IPv4 optional in most of these cases).
I can accept either a dual-stack library or an IPv6-only drop-in replacement for the current LWIP library, but I need something that supports IPv6.
While my customer may be on the leading edge for this need, the reality is that the world is almost completely out of available IPv4 addresses (ARIN has fewer than 65,536 remaining in their free pool, RIPE, APNIC have been out for years, LACNIC is basically out, AfriNIC has a free pool, but only because IP deployment in Africa is lagging the world).
IPv6 is necessary for any product to have any longevity. Especially in the IoT/connected sensor world where the ESP8266 is so ideally suited.