I think Matheson was pretty overrated here. He skated really well and that was about it. He was a shitshow defensively, and didn't really provide offense to the degree you're really looking for as a second layer behind Letang.
Marino was pretty shit here outside of his first 50-60 games in his rookie year, but it's becoming pretty apparent that it was likely due to coaching influence rather than Marino just being a flat-out turd. Seems like the FO moved him because they needed to clear a bit of cap and the asking price to take on Petts wasn't palatable and Dumo was never even considered to be on the table despite a steady decline in play in recent years.
I've never been a fan of Petry's, and would cringe whenever he was brought up as a potential Letang replacement if he'd left this past summer. I think he's way too wild in the pendulum swings of his game. He's a bit of a lazy dipshit, he's prone to dumb penalties, and is just as much a clownfest in his own zone as Matheson was. He's not as good of a skater or in transition, but he provides a bit more physicality and he's historically been better at directly producing from the blueline and I think that was the main goal--to try and get somebody to reliably be a threat from the backend when Letang wasn't on the ice. Just hasn't worked out thus far.
I'm still interested in seeing how Smith develops. A Smith-Rutta bottom pairing seems pretty promising because it'd get sheltered minutes and Rutta is a fine partner for a dude who is way more offensively capable and focused like Smith. It's just gonna be a bit of an issue to sort out the rest of the blueline to fit that bottom pairing in. You're gonna need somebody more defensive-minded to handle being Petry's babysitter, and I'm not sure POJ is that guy. If anything, I think you have to look to dump Dumo and promote POJ to play with Letang full-time in a sink or swim move. He certainly can't be as bad as Dumo's been. I just don't think the FO or coaching staff see Dumo as the problem he very clearly is. I don't know that Dumo would be super sought after with his play recently, but GMs do love a resume and Dumo's has "2x Cup winner, defensive-minded vet, top pairing minutes" so I'm sure some doofus out there would be willing to take a chance on him, if not give something decently valuable in return. I hate to bring up Scuderi-Daley because that was such a wildly anomalous situation that I don't think it'll happen again, but if end of career Scuderi can be dealt, and dealt for a good return, then anything's possible I guess.