It's a bit difficult to argue that a player has yet to fulfill their potential after winning two Norris Trophies and leading the league's defensemen in scoring three of the previous four years.
A bit difficult, but not impossible.
Last year, Erik Karlsson was anchored to various bottom pairing defensemen (at best) for half of the season and he still managed to be granted the league's highest praise for a player at his position. It was miraculous, and nearly unbelievable at the time.
After the performance he put on last year, it almost seems arrogant to predict the best is yet to come.