I agree with a lot of what you said in the first half of your post. Petey definitely isn't the same caliber as Mackinnon (only other player in that realm is McDavid).
Yes Petey's line has gone down in minutes, but it also makes sense when you look at strategy and deployment. The third line has been shutting teams down while being able to produce. In the playoffs, you don't really get the luxury to wait for guys to get going. If you're not going to produce, you can expect your minutes to get peeled back.
In regards to strategy, playing defensively was definitely the right move. As great of a story Silovs and to a lesser extent, DeSmith, the team had to adjust their play to limit scoring chances. Without that defensive structure, Silovs would've been blown out no doubt. The regular season LA Kings shutdown play has shown to be effective against Vancouver and there is 0 chance Nashville wasn't aware of that. Instead of trading chances, playing the same but better game was definitely the right choice tactically.
No doubt, Toc will come out with a different strategy against the Oilers.