Reason for optimism: Seattle has aggressively addressed the weakness that has annually crushed the team's results during the Felix Hernandez era.
Reason for pessimism: Hernandez turns 30 this year, Robinson Cano is 33, the Dodgers were starved for rotation help and passed on Hisashi Iwakuma, etc. The window for this era's success is close to closed.
It escaped the eyes of even the sharpest baseball fan, but in 2015, the Mariners embarked on a very peculiar experiment. Despite playing in the American League, they eschewed their right to use a designated hitter for the entire season. I know what you're going to say, "Joe, I watched the Mariners play last year, and I don't ever recall pitchers hitting in any but a handful of interleague games." That would be another example of how our eyes lie.