I really don't see why people insist Hornqvist is a must sign player, he's just not. If you can't find a spot for him at ES, which seems kinda likely based on who else the Penguins have on the roster, you don't pay significant money into keeping him. It's just being way too attached to those players. If Hornqvist would be willing to sign a 3 year deal at $5 million, I'd make space for him. However, I'm not going to match whatever he's going to get from other teams just to keep him. That would be a horribly shortsighted move, it puts the Penguins in handcuffs with the cap and roster management long term. What Hornqvist brings to the table on the powerplay is nearly irreplaceable, but the Penguins just have too many other good RWers to justify giving him a huge contract. That doesn't even mention Hornqvist's bad fit with Malkin and Crosby not liking playing with Hornqvist, either.
If you re-sign Hornqvist for $6 million for 6 years (which is very likely similar to something he'll get in free agency), you're locked into a $6 million guy on your 3rd line for 6 years. Just like you can't justify paying Bonino $4.1 million to stay here as the 3C since the Penguins don't have anyone to internally replace him, you can't justify paying Hornqvist $6 million to stay here as the 3RW.