I'm not sure I like the idea.  We aren't forcing our users to use the new format, but we aren't encouraging it either.

If our "legacy" users are using a newer version of Logstash, they can use the codec to convert the old format into the new format on the fly.  We want them to upgrade to the newer experience.  My personal feelings on the matter are that we give them every incentive to switch, rather than make things backward compatible.