This has been tried before in the past. We had to rollback that change because people with broken IPv6 setups (read: that have IPv6 but it isn't working properly) couldn't connect to anything. Besides, Mojang's auth servers are IPv4-only, so it's not like IPv6-only connections can even authenticate on the launcher.
Also, as far as Minecraft servers running on IPv6 addresses goes, Minecraft itself couldn't connect to IPv6 servers until a certain version.