Why would it hurt his legacy? Because he tried to actually win the fight instead of being used for target practice by a superior striker? Makes no sense to me.
It will leave it's legacy where it is. If Bonnar goes right in to tie up, he's have the same standing after the fight as when he went in, and in other words got beat up for nothing. If he tries to grapple so much that he looks like Maia or Leites did, then it will even take away from his legacy. This isn't a fight Bonnar will win by grappling. Someone doubted it earlier but he has a punchers chance and really the only times Silva took some punches, against Sonnen, he didnt take them all that great. If Bonnar walks through everything Silva has, proves his toughness and grit, that replaced a lack of HOF worthy skills or results and might just make him a real legend of the spot. This is his chance to cheat the system and if he tries to grapple his way through it, he's going to retire as "That guy who had a dope fight with Forrest".