Toronto's heavy reliance of Matthews on the PK isn't because Toronto has so much trust in him (though they definitely do), but because they don't have anyone else to kill penalties to the level of Matthews. Sure,
William Nylander could be a new addition to the PK but just like his turn at center, if Craig Berube tries it, will it actually stick? He's desperate for more minutes, and
Mitch Marner isn't necessarily the best PK option, GM Brad Treliving needs to find a true 3C who can kill penalties on the first line and give Matthews a break.