Deferring to other's expertise on how the NHL salary cap and compliance buyout process work, here is what I think I know.
1.
https://en.wikipedia.org/wiki/Compliance_buyout The compliance buyout still exists. My thinking was/is that each team got/gets one buyout -- the Pens have not used this previously -- the Pens could use it for Letang or Phil if it still exists?
2.
https://www.capfriendly.com/teams/penguins Phil & Letang are now 30-something with 4 years left on their deals. Combined they would make north of $14 million EACH SEASON during that time. I can't see both being here for the next 4 seasons -- frankly, I'd say both will be gone by then.
3. By way of comparison, Hagelin and Brassard are both of similar ages, but only have one year remaining at a combined salary of half, or $7 million. This actually makes Hagelin and Brassard more attractive potentially to a team that wants to win now, meaning the Pens may get a better deal for 1 or both of them than for 1 or both of Phil/Letang.
So, what does this all mean -- I'll play amateur GM and capologist.
1. Phil gets traded this summer, maybe at the NHL Entry Draft for picks. The picks would help, but dumping some/all of that contract would be the main attraction for the Pens. I would rather keep Phil than Letang but I'm reading Sully's tea leaves.
2. Letang stays around another season and then gets traded or bought out if he doesn't up his game. I agree off his recent performance combined with his salary that the Pens wouldn't likely get top value by trading him now.
3. If they can't trade Phil or Letang then Hags gets traded at the Draft. I like Hags and he's valuable on the PK, but he doesn't score enough. Brassard is seen as a Swiss Army Knife -- can play 3rd/4th line center or fill-in on the top 2 lines if Sid/Geno get hurt. Also, Hag's $4 million plus the increasing salary cap can be used to find another FA plus to pay Jake whose going to need paid soon.