Title basically says it all, this is an awesome dungeon and it would be a shame if it just disappears after the event is over. On the other hand if it stays where it is and they made it into a permanent dungeon it would just feel like another CoF.
I think they should truncate part of it, keep the final boss and maybe the training room with the core and turn it into another fractal. That way FotM will be bigger AND we get to keep a cool dungeon they put a lot of hard work into.