I think Babe Ruth had 15 good years as a batter, then was not that great his last year and change.
Gehrig had 12 years+ years he was playing well.
Ruth's physical shape went off a cliff at 38. Gehrig was 35 when he was forced to stop playing. He was certainly in much better shape than Ruth. But it's hard to say, even some healthy players hit a wall in terms of performance.
If Gehrig had played 5 more years at 40 HR per, he’d still come up short of Ruth. There was no way he catches Ruth.
Interesting that prior to the announcement of his illness, Gehrig was catching flack from some sportswriters for his declining play.
Finally, there have been some medical opinions offered that Gehrig was not afflicted with ALS, but instead brain trauma from playing football for years, essentially without a helmet. He apparently suffered several (many?) concussions.
Gehrig followed Ruth in the batting order (imagine what an inning like that was like for an opposing pitcher!). Here is what “Murderer’s Row” typically looked like
Combs (.356)
Koenig (.285)
Ruth (.356)
Gehrig (.373)
Meusel (.337)
Lazzeri (.309)
Dugan (.269)
Collins (.275)
Pitcher
It looks like it would have been a rough day. Poor Mark Koenig. I imagine he was seeing the pitcher’s absolute best (you didn’t want to put any “ducks on the pond” for Ruth/Gehrig). Dugan and Collins are the weak links...and every fourth time up, they were reaching base.
There are some projection methods that try to determine how a player would have done in his career had he a normal decline period and his career had not been interrupted. They were designed primarily for the players whose career was interrupted by WWII but theoretically should work for players whose career ended prematurely.
Bill James ran the projection for Lou Gehrig: 689 home runs, 2,879 RBI, .330 BA, and 3,928 hits.