According to some of the least trustworthy writers on the internet (e.g. Heyman), the Yankees are interested in Freddie Freeman! On one hand, that's great; he's one of the best players in baseball, he's a lefty who hits for a unique combo for power and average, and he can be had for only money. On the other hand, he's 32 and apparently wants a six year deal. That's a scary age, so it'd be nice to at least approximate how he's likely to perform as he gets to a point in his career when a lot of players turn into pumpkins.
Luckily, there have been a lot of elite 1B in the past 15 years, so we can use relevant comparisons to benchmark how Freeman will perform over the next six years. I used the following criteria to generate a relevant list of comps, based on Freeman's own performance in the past 3 years:
1. 10+ WAR in ages 29-31 seasons
2. 130+ wRC+ in ages 29-31 seasons
3. .275+ batting average AND/OR <20% K rate in ages 29-31 seasons
4. 75+ HRs in ages 29-31 seasons
Those four criteria give us a set of seven relevant comps. For each, I've shown below their stat line from ages 29-31, and their stat line from ages 32-37 (the timeframe for Freeman's expected contract):
29-31: .329/.401/.587, 169 wRC+, 21.1 WAR, 7.0 WAR/season
32-37: .293/.374/.470, 126 wRC+, 10.0 WAR, 1.7 WAR/season
29-31: .313/.409/.598, 164 wRC+, 19.1 WAR, 6.4 WAR/season
32-37: .262/.319/.459, 112 wRC+, 7.0 WAR, 1.2 WAR/season
29-31: .308/.404/.560, 150 wRC+, 16.3 WAR, 5.4 WAR/season
32-34: .243/.350/.424, 111 wRC+, 5.0 WAR, 1.7 WAR/season
29-31: .282/.379/.523, 134 wRC+, 13.4 WAR, 4.5 WAR/season
32-33: .296/.378/.503, 140 wRC+, 10.8 WAR (with 2020 season prorated over 162 games), 5.4 WAR/season
29-31: .266/.363/.514, 131 wRC+, 13.0 WAR, 4.3 WAR/season
32-36: .229/.322/.442, 108 wRC+, 5.1 WAR, 1.0 WAR/season
29-31: .274/.370/.546, 149 wRC+, 12.2 WAR, 4.1 WAR/season
32-37: .253/.352/.511, 129 wRC+, 14.1 WAR, 2.4 WAR/season
Lance Berkman (split time between 1B and OF)
29-31: .295/.405/.552, 144 wRC+, 11.6 WAR, 3.9 WAR/season
32-37: .280/.394/.495, 138 wRC+, 16.5 WAR, 2.8 WAR/season
Whew, that's a lot of numbers. Overall, our seven comps averaged 5.1 WAR per season between ages 29-31, and only 2.3 WAR between 32-37 (1.8 WAR if we exclude Goldschmidt, who's only played through age 33). Similarly, they averaged a 149 wRC+ between 29-31 (unweighted average) and only a 123 wRC+ between 32-37. That's a drop of 2.8 WAR and 26 points of wRC+.
However, not all players age identically. Pujols, Youkilis, and Teixeria declined quickly and enough to have effectively no value (~110 wRC+ 1Bs are a dime a dozen) whereas as Encarnacion and Berkman had longer peaks and remained productive hitters through ages 35 and 36 respectively. Cabrera was a bit of a mixed bag; he was spectacular for ages 32 & 33 (165 & 153 wRC+), but fell off quickly after, struggling to remain even a league average hitter.
Overall, if we apply the average decreases to Freeman, we're looking at an average of 2-3 WAR and a ~120 wRC+ over the life of the contract. He'll probably remain very productive for the first couple of years and then decline significantly. Best case, he'll be like Encarnacion or Berkman and hit until the very end, worst case he'll completely fall of by 33 like Youk or Tex. Not exactly groundbreaking analysis, but it's at least comforting to know that not every 1B turns into Pujols.
Given his relevant comps, I still lean towards signing Freeman. The Yankees' window is wide-open now, and they desperately need another great hitter while Judge and Gallo are Yankees and Cole and Stanton remain in their primes. Freeman is that hitter, and as history shows, he should remain that hitter for at least the next two years. After that, it's anyone's guess, but if the next generation of prospects hit, his decline will matter less, and if they don't, then the money we save by not having Freeman probably won't be able to save this team anyway. The Yankees should be maximizing for 2022 & 2023; Freeman is the best way to do that.