You might be able to win with six Letangs, but Smith isn't Letang. Letang, for all his brainfarts, actually plays defense. He ties guys up, boxes out, finishes checks, clears the puck, etc. He's played major PK minutes on a unit that's been pretty effective all season. Yes, forwards like Acciari make a difference, but so does the back end on that unit.
The fact that Ludvig, Shea, Ruhwedel are underwhelming as bottom pairing defensemen is an argument that they suck. It has zero bearing on the concept that you need variety and balance to win.
You need skill and offense, but you also need guys who are willing to eat shit minutes and prioritize defense. Finding that balance is tricky, and it looks like the Pens went too far in the wrong direction, but it's absolutely how you win.
No team has won the Cup recently with a blue line of Letangs or a forward group of Guentzels.