Crosby and Malkin alone are enough to make them a playoff team, but they aren't enough alone to make them a contender.
In truth, they never have been, cause there was a 7 year gap between 2009 and 2016 where the Penguins failed to provide them with enough support to be much more than playoff team. Some of those years you can toss out as years injuries killed them, but there were several years in there where Crosby and Malkin were going, dragged a team into the playoffs, but weren't enough by themselves to do much there.
That's not a knock against them, that's hockey.
The good thing for the Penguins is, that Crosby and Malkin are so good, that even if they are beginning or in the midst of decline, they're starting from such a high that even at 80% of what they used to be, they're still great players. So that basically buys Pittsburgh more time on their aging curve than teams built around less amazing players have to work with.
Improve the depth again like in 2016 and they're good. The longer you go, the better that depth has to be, because the longer you go, the less and less (however gradual), Crosby and Malkin can carry themselves. The window isn't closed, there's just more urgency.