We don't tend to talk much about the sizes of the various positional adjustments, but in my opinion, the accepted positional adjustment of -17.5 runs for DH as compared to -12.5 runs for 1B doesn’t make much sense. I suppose we want to assign some sort of bonus for at least being coordinated enough to have a defensive position, but when you dig deeper, I don’t think the 5 run drop from 1B to DH holds up.
Positional adjustments exist because players generally need to excel in both hitting and fielding skills, and since those skillsets are largely independent of one-another, players that are great in both categories are exceedingly rare. Assuming complete independence between the skillsets, 10% of the population will be above the 90th percentile in hitting skill, and 10% will be about the 90th percentile in fielding skill, but only 1% will be aboth the 90th percentile in both skills. Since you need an elite defender at SS, the defensive requirements limit the available population so much that you tend to cut out almost all of the top hitters. The same phenomenon happens to some degree at catcher, CF, 2B and 3B.
The result is that more difficult defensive positions tend to be populated by better defenders who are generally poorer hitters. Meanwhile, the easier defensive positions (1B, DH and corner outfield) don’t place stringent limits on defensive ability, so there’s a large population from which to draw the best hitters.
The positional adjustments are thus formulated to credit players for the scarcity of their defensive value. It’s hard to find a player with enough defenisve value to play SS, so a player that plays SS gets a big fat positional adjustment. At the other end of the spectrum, literally every person on earth has enough defenisve value to play DH, so we dock DH’s a big chunk of value.
But in order to justify giving 1B 5 more runs of positional value than DH, we need to believe that the 1B position sets the defensive bar sufficiently high that it meaningfully limits the number of high-level hitters that can play the position. And I don’t buy that it does.
First, we should note that hitting and defensive skill isn’t totally independent of one another. Guys that are athletic enough to hit a 95 MPH fastball over the fence are almost always athletic enough to play 1B competently. Second, we should also recognize that there are 30 1B slots and 14 DH slots in MLB. So in order for the defensive requirements of 1B to impart meaningful defensive scarcity, 15 of the top 44 1B/DH hitters in baseball need to be sufficiently clumsy that they can’t hack it at 1B.