Penguins' GM Kyle Dubas isn't expected to get a big return in any trade with the Penguins this season, which is reportedly why their veteran trio hasn't been moved yet. If a deal happens with the Golden Knights, Dubas likely won't be impressed either.
For the Penguins' future, trading Karlsson now makes sense to gain assets and clear cap space. If the Golden Knights view him as a key playoff addition, expect them to aggressively pursue him. Karlsson has also been linked to the Senators, Hurricanes, Stars, and Red Wings.
Ultimately, Karlsson's best days are behind him, and the Penguins will likely need to retain a significant portion of his salary to facilitate a deal. But if they're motivated and Vegas stays in the mix, don't be surprised to see a trade come together in the near future.