The three years differential happens to be the most crucial 3 years on the age curve dynamic. 32 to 35 is when most players show steady decline or drop off a cliff near 34-35-36 years old.
Fangraphs has these numbers on Smith that clearly show he is in decline and already under his career norm for the last two years:
Last 3 year trend (age 31-32-33). He turns 35 in Sept.
wRC+: 131> 113> 110 (career 113)
wOBA: .357> .331> .331 (career .343)
Baseball Reference has this:
OPS+: 134> 116> 108 (career 112)
Since Smith has always had a "pitcher's" home field (OAK, SD, SEA) choosing stats that adjust for this all show a decline. Maybe moving to BAL will improve his OPS and SLG numbers, but I wouldn't expect and increase in any of the 3 stats I just provided.
Moreland's numbers have been all over the map with some massive swings up and down. Choosing a 1 year decline to try and show his trend is downwards is not really fair. His age is also within the range of prime, so there should be no age-related downward projection either.
Look, I wouldn't bet on Moreland equallng or bettering Smith in 2 of the 3 stats I used above in 2017, but I think there's a good chance he does, and as you pointed out, the defensive value Moreland brings to the Sox infield, and the steep decline in Smith's defense makes the Sox choice the right one. The money differential between trading Buc for Smith vs trading Buch for no cost added and signing Moreland instead is icing on the cake.