Honestly, I'll make a steel man argument for the 4 lines:
First line: Laf and Panarin have carried their line this year, I don't think Trocheck has been quite as good. Since they drive play anyway the downgrade won't be as significant. And really Mika SHOULDN'T be a downgrade. Maybe getting Mika away from Kreider will do him good.
Second line: Cuylle has been a beast but if you think Kreider-Mika have been toxic Kreider, who plays a north south game and is a better finisher.
Third line: Trocheck hasn't been as good this year but Cuylle and Smith have been good, if the top 6 works the is legit a very good 3rd line. I doubt it's as effective as the Chytil line was, but hopefully we get more production from the top 2 lines.
Fourth line: They've been good and are getting an upgrade in Vesey.
The D sucks, but with balance on offense I really really hope that and Igor is enough. The Panthers didn't have superstars on D and neither did the Pens beyond Letang.