Having your core as v6 only lets you push NAT to limited places (one of the many options for 4x6x4 NAT, including stateless options if you're willing to cut certain corners off v4).

And v6 connections help drop the pressure on NAT resources - and sites that are optimizing for mobile connections are already going to be on IPv6 where possible (due to mobile networks prioritizing v6 traffic for various reasons, including licensing - and NAT resource costs)