Maatta has had a hell of a start but he's far from a sure thing considering the past few seasons. His injury history is still a tad concerning.
Maatta isn't completely replaceable per se--especially if he keeps his 82 point pace-- but the Pens won last season with him at 60% arguably. You're telling me you couldn't fill that hole with one of the guys Vegas is trying to rid themselves of?
When Pen fans talk about being able to replace Maatta, they're not talking about his offense (and specifically his PPG pace as they do not expect that to continue, but his overall game (specifically defensively and his puck movement). Maatta might only be playing 18-20 minutes a night, but as we've seen come playoff time, he's easily capable of playing 22-23 minutes a night against high end opponents. Getting someone like that isn't possible with the D Vegas is offering up. Garrison would hold some appeal, but would likely be expensive to acquire.
And while he's had a few injury issues over the years, that's not really a concern that any PIT fan has with him. However it is something we have to worry about with Letang... so while Maatta might be a luxury today, all it takes is a Letang injury (and that's pretty much a given every season) for him to become a necessity.
So I go back to my initial point... Pittsburgh doesn't need a young stud as a #3C, when there's other options available that will not require them to give up a key player to their blueline. So while I'd love to get Reinhart, it doesn't make a ton of sense right now, but I'd be willing to reconsider things in the offseason, when we again have the chance to potentially replace Maatta in FA.