The biggest problem is the age of the core, period. Dubas did his best to stave off the inevitable rebuild and no, not all great moves, but no doubt his mandate from ownership was to try anyway. This season will see the start of the great tear down, gonna be a long decade for Pen's fans, buckle up
You keep saying this but this just isn't the case. Outside of Karlsson (who I'm not sure I'd call a part of the "core" anyway), all of their core players are performing up to their cap hits. Letang is a bit messy, but he's only making $6.1 million a year. Crosby, Malkin and Rust are all producing enough to warrant their cap hits.
The issue is that this team has nothing around the core, not that the core is old. The Crosby/Malkin/Letang/Rust/Karlsson core is making like $36 million, and I firmly believe that they're outperforming that total money (even with how overpaid Karlsson is). The issue is that they have no young players and no prime aged players worth a damn along with having a coach that is about 5 years past his expiration date.
That's why I keep bringing up JR and Hextall as also responsible for the Penguins current situation. JR traded away pretty much every 1st along with about 2/3 of 2nds and 3rds from 2014-2021. You simply cannot build a prospect pool with that few draft picks. Hextall then made it worse by giving away numerous prime aged players (namely Matheson, Marino, McCann and Rodrigues) for nothing and committing to older and worse players (namely Carter and Petry). Dubas has made things worse since coming here but he inherited a horrendous situation.