Before the 2016 season,
Speier wrote an article about Bogaerts and Betts and how a lot of similar young players had a fall-off in their age 23 seasons. But Mookie actually got 3.5 WAR better in 2016, while Xander did drop, but by less than 1 win.
The season where they both had a big dropoff came in 2017, when Mookie dropped by 3.1 WAR and X dropped by 1.5.
2017 was a bump in the road on their development, but development is very rarely linear, as Hazen said in that article:
"General manager Mike Hazen, while praising his young players, offered a measure of caution.
“Look, there’s nothing that says these guys are going to continue on a linear path upward,” said Hazen. “It doesn’t work that way.”
Indeed, history suggests that even for elite young players, the arrow doesn’t always point up. Bogaerts and Betts became the 74th and 75th middle-of-the-field players ever to post a WAR of 3.0 or better in their age 22 season.
Of the previous 73 (a group that included 26 Hall of Famers), decline at age 23 proved a common occurrence.
While 27 of the 73 players (37 percent) maintained their level or improved, 46 (63 percent) suffered some decline at age 23, with half of those players enduring declines of 2.1 wins or more. On average, the group of players who were worth three or more wins at age 22 saw their value drop by 20 percent (and 1.0 wins) at age 23.
The reasons are numerous: Injuries, a normal dropoff in production, and position changes away from the middle of the field (as with Betts moving from center to right) can all diminish player value.
Regardless of the cause, the idea that players might be less valuable at age 23 than they were in standout age 22 seasons can’t be ignored. After all, 10 of the 26 future Hall of Famers on the list – players like Joe DiMaggio, Bobby Doerr, Paul Molitor, Robin Yount, and Tris Speaker) endured a decline in value at age 23 of a full win or more.
Yet even with a 20 percent decline in value, both Betts and Bogaerts would remain standout players. Last year, there were five shortstops besides Bogaerts to post a WAR of 3.5 or better in the majors. If Betts’ value falls 20 percent to a 4.8 WAR in 2016, that would still put him in the company of the top 33 position players in the majors last year, just below the likes of Andrew McCutchen.
In short, Bogaerts and Betts represent building blocks precisely because they stand a strong likelihood of being elite players even if they are not as valuable as they were a year ago."
So these guys having a dropoff for one year while they were younger than 25 to me doesn't seem like something permanent at all, just something that often happens with young players. These guys happened to have their drop-off seasons at age 24, rather than age 23. I personally expect them both to bounce back in 2018, depending on health as always.
Seems like a lot of people have decided that X is always going to be the same level he was in 2017. Personally, I am hoping he doesn't get hit in the wrist with a pitch again (he got hit in the wrist April 2016 as well as last season) and hoping that being another year older and working with a new hitting coach might get him driving the ball a lot more than he has in the past 2 seasons.
Their Steamer projections seem perfectly reasonable to me, and I think they'll both be around those numbers if they stay healthy.