I agree that playing playoff style hockey, like old school shut them down and make clutch plays out of the corners etc, is something that McDrai just dont do... and it WOULD be a positive for them if they did play more like Barkov (so I largely agree), but we have seen other stars that dont play that style and succeed in the playoffs too.
Ultimately I think even if you replace McDavid and Draisaitl with slightly better Barkovs... they lose the series. The rest of the team would still lose their matchups. They would be a better team with 2 Barkovs but they wouldnt be good enough to have beaten that Florida team. The real key is that 2 Barkovs would give you the cap space to get a real goalie...to get guys who care more about playing their role and helping their team than about other nonsense.
That all said, I do agree with you and I would honestly have massive respect (moreso than I do) for McDavid if we saw him take his matchup and grind that out and get in lanes and backcheck hard and be relentless on the puck everywhere... that wouldnt be enough given how other guys played. Maybe you are right that if they see McD and Drai doing that the other guys match that energy and the cultural shift in the locker room puts them over the top. But I am not sold on that.
I am on the side of (and could be convinced otherwise) that they'd be better off with only one of McDrai. That would take some good GMing though and I am unsure they have that.