Even if Nyquist and Tatar have plateaued, I don't think all of Sheahan, DeKeyser, Ouellet, Marchenko, Smith, Pulkinnen, Mrazek, Jurco, and Glendenning have hit their peak. Add in Green and Richards and I highly doubt they regress without significant declines/injuries to D and Z. Other wildcard possible impacts are Franzen and Larkin.
I agree that coaching is a big variable. We'll have to see how that plays out but even if Babcock is the better coach I don't think the coaching change necessarily hurts. Sometimes a coach'a message runs thin and the young players could respond very well to playing under Blashill again. Remains to be seen.
Obviously significant injuries throughout the lineup will derail a season, but that is true of every team.