My guess is to make upgrades easier.
You can upgrade to the 3.0 binary without needing to migrate data to the new storage engine, which takes time.
It wouldn't make sense from an ops standpoint to make an engine-migration mandatory.
You can upgrade to the 3.0 binary without needing to migrate data to the new storage engine, which takes time.
It wouldn't make sense from an ops standpoint to make an engine-migration mandatory.