It's beyond one year, and yes, no one is listed on the roster as DH. But that's not what I mean. People think a team's "Starting Lineup" plays 150 games. They really do not. You'd say JD Martinez primary role is DH, and that's true. But the Red Sox had the same defensive alignment including DH exactly 12 times last year. Martinez DH'd 107, and 9 other guys the other 55. He played 24 games in RF, and 13 games in LF. THAT'S what teams do now. They get 7 guys for 5 positions and rotate off days and DHs. The Yankees used 10 guys at DH, none more than 34 games.Also your other assessment is strange, as it's just a 1 year assessment (and therefore could be an outlier) and ignores the fact that teams bring people in as "primary DH" and pay them money for it. No one is going to confuse JD Martinez's primary role on the Red Sox with being Outfield instead of DH. Sure, technically he's listed there on sites that only list "Pitchers" "Infield" and "Outfield" for groupings, but he's not a starter out there.
Here's the number of DH's who qualified for the batting title by year:
2019: Cruz, K.Davis
2018: Cruz, K.Davis
2017: Cruz, Encarnacion, Morales and Pujols
2016: Ortiz, V.Martinez, Morales and Pujols
2015: Ortiz, Morales, Fielder, A-Rod, Butler, Gattis
In five seasons, we've gone from 6 to 2. Teams want flexibility to match up more now. Half of the 14 total players listed have retired, two were on the SAME TEAM last year, and while every year everyone gets a year older, pushing guys from from "Primarily Outfielder" to "Primarily DH," it doesn't alter the fact that the high priced "extra hitters" are involved in platoons; MLB is trending younger; and a DH for SP only list doesn't remove them from the game.
Nelson Cruz is the only player in baseball who ONLY was a DH last year. He can still DH for the SP and bat third for the first six innings (which isn't that different from NL baseball. Guys are replaced for defense late all the time; Jose Martinez finished 31 of 79 games started and Tommy Edman finished 52 of 82 games). But it makes every team in baseball decide if you'd rather have Nelson Cruz types hit in innings 1-6 and not be available late, or if you want to save Cruz for off the bench in a bigger spot.
And of course, it makes every team use their bench more. More opportunities per season for more players means more guys show their value and get contract offers. The average AL team only used a pinch hitter 77 times each last year. The NL 255 times. This would basically create 3000 more bench ABs per season. And it's FUN late in close games or extra innings.