Based on achievements and trophies statistics, most players absolutely do not do optional content, and most don't even finish games.They are, but the issue is that they are available to do before you beat the game. A newcomer who's never played FF1 isn't necessarily going to go in the game with the knowledge that they need to avoid these dungeons. And most players do the optional content in games. So you will have situations where new players are going to unlock these dungeons, and immediately go do them, which will make some of most of them go "what the hell is this?", which in turn might get them to stop playing the game. And we don't want that, since FF1 is a good game.
The only way I would be fine with the inclusion of the dungeons is if they unlock only after the credits have rolled. Other than that, I recommend excluding them.
That said, I'm all for having the extra bosses selectable from a menu for the player to fight.
But yeah i'd be fine with extra bosses being selectable from a menu as well. Also to the person saying well 2 through 6 are already gonna have these bosses, not really the point of fighting bosses not originally in a game from another.