The Athletic's Chad Jennings has written an interesting article (subscription required), detailing the biggest offseason change by every American League team. For the Mariners specifically, Jennings noted how they are now building beyond the rotation, by focusing more on the lineup and looking for ways to strengthen the bullpen.
However, this doesn't change the reality that the rotation will still play the biggest role in helping the Mariners remain contenders, as they look for the first World Series berth and championship in franchise history. And that window for contention looks set to be open for a while yet, when considering the age of the current starters outside of Luis Castillo, combined with talented prospects such as Kade Anderson and Ryan Sloan.