Because it could be universally panned and they would lose their audience within one episode.
A fresh reboot with a fresh story and a fresh cast on a fresh new day could also revive the entire series. Everything is good about that if it does. Optimistically, I am trying to see this as Once’s last chance to stand triumph despite all its mistakes. Getting rid of the cast is a big, edgy move but who knows how it will all work out. This very well could be the most captivating season of OUAT to grace the screen with the most interesting and shocking character arcs.