I'm honestly surprised the Flyers haven't shown any interest in Karlsson. They have a top 3 prospect pool so its not like they don't have the assets to make a deal
There are arguments both ways. They have 2 players already making more than $8 million, a whole host of young defensemen and forwards that will need new contracts in the next 4 years, not to mention Simmonds, and they have a similar (inferior) player in Gostisbehere already, signed long term at well under half of what Karlsson would want. Their 2 small defensive gems (Ghost and Provorov) have already proven themselves, but the big guys (Morin, Sanheim, Myers) are all still question marks, so they may not be after another small offensive D-man (even if he's the best in the league) at the price he'll ask, both in property and contract money. Also, Hextall is a really conservative GM.
That said, they do have that great prospect pool, and their cap situation isn't bad at the moment. They have both Couturier and Gostisbehere signed for the next 5 years at $4.3 million and $4.5 million respectively, which is insane if both can sustain their current play. Giroux and Voracek are expensive, but the Penguins have shown that you can make a topheavy roster work.