Here are the inning totals for 2015. Notice, the lack of workhorseness from pitchers not named Felix.
In 2015, the league average OPS against for pitchers was .723.
The only guys who beat that were Felix, Iwakuma, Walker and Paxton for the rotation, and Furbush, Smith, Zych, Kensing and Wilhelmsen for the bullpen. The good relievers only pitched 161.333 innings. The good starters only pitched 568 innings. That makes 729.333 innings of above average pitching for the Mariners. This left 734 innings of below average pitching. Smith pitched the best of any Mariners player. 11.8 K/9 vs. 2.8 bb/9. 2.32 ERA over 70 glorious innings. This is only slightly worse than the year Craig Kimbrel had.
The new guy, Wade Miley, is a proven 200 inning man. He as a .732 lifetime OPS against, despite playing in hitters parks in Arizona and Boston. His lifetime ERA+ is 101. This guy is metronome league average.
The value of him is 1. that you take the 200 innings that he pitches, and erase vidal Nuno and J.A. Happ from the roster. Nuno was a meatball, and Happ was okay, and 2. The bullpen gets a rest on days that he pitches, which means that you have fewer flameouts like Danny Farquhar and Fernando Rodney. The 2015 Mariners bullpen was definitely overtaxed. The 2015 Mariners bullpen was definitely bad. Baseball guys say the two are correlated. If there is no proven player behind Felix, the Mariners are set up for failure again.
Miley is a good pitcher. I don't know if he is good enough to give up a league minimum 12k man Carson Smith for, but he is a good pitcher. The Mariners needed someone like him. There is a case to be made that he is much better than Roenis Elias, but there is no way that Smith will not be missed. At least we still have Furbush.