It most likely will be on by default. They have to implement it with some kind of default setting and that makes the most sense, given the purpose behind it. No reason to get upset about it. Just go switch it off when it comes if that’s your desire.
From the FAQ:
“It is technically a much more difficult task to do so because the stat system would need to be split into two separate progressions that would read from separate tables. Additional resources and time would need to be spent on maintaining both new followers and legacy followers. We felt our efforts were better spent on adding new content and systems into the game than spending those resources on legacy followers.”