*This is unfortunate, but not nearly as unfortunate as some people are going to claim. MLB draft picks are a step above total crapshoots. Free agents are much better bets to produce at the MLB level than any draft pick.
The 2015 Mariners were not all that different from the 2014 Mariners, believe it or not, despite a season that feels very different. The reality is that the 2014 team played better than their .500 talent, thanks in large part to a superhuman bullpen. It is also true that the 2015 team had roughly .500 talent and they squandered a number of games. Bullpens giveth and taketh away.
Here are the 2015 and 2014 Mariners were by WAR:
Top Three SP
Rest of rotation
Better Offense: 2014 by 1.8 WAR
Better Rotation: 2014 by 1.8 WAR
Better Bullpen: 2014 by 3.3 WAR
The 2014 Mariners won 11 more games than the 2015 Mariners with about 6.6 WAR more talent. WAR doesn't equate perfectly to wins, but it's the most perfect conversion of individual talent to team wins that we've got. Like I said, the 2014 Mariners played beyond their abilities by just a bit, and the 2015 Mariners played beneath them by just a bit. The end result feels like a large chasm but to some degree that is a mirage.
A difference of 1.8 WAR for an entire unit, like a lineup or a rotation, is razor thin. For instance, Robinson Cano was worth over 3 WAR more in 2014 than in in 2015. Kyle Seager also was over 2 WAR worse in 2015. Either of those players (not even both!) looking more like the 2014 versions of themselves would have wiped out the gap. Heck, even Mike Zunino was over 1.8 WAR better in 2014. These aren't even individual players taking steps forward - just not taking steps back.
As for the starting rotation, King Felix had an off year. He was worth over 3 more WAR in 2014. A return to form for him would have made for a 2015 starting rotation that was superior to 2014's.
So, depending on how you feel about the Mariners core players moving forward, it might not be too hard to imagine this team getting way better in a hurry. Jerry Dipoto will inevitably receive significant praise if/when the Mariners improve in 2016, and perhaps he will make some magical moves, but the 2015 Mariners were never as far away from contending as they seemed. For whatever reason, things just went wrong for them. The American League had lots of .500-ish teams, the Mariners being one of them, and a few subpar performances from key players were enough to sink them. That's what it meant to be True To The Blue** in 2015.
**Had to use the slogan one more time before the marketing department makes something new for 2016. The irony of this slogan might be what I remember most from 2015.