Your post caused me to look up Goodman's stats. His power really did do a nose dive after 1938, when he hit 30 HRs to go with a .292 average. Those 30 HRs ranked second in the NL that season. He also had a .533 slugging percentage for 4th in the NL, and was an All Star. Goodman was also an All Star the next season, but while he hit .323, his HRs fell to 7.
However he was also age 29 that season, in an era when players tended not to remain at an elite level for as long, so he perhaps just had his peak season before age took its toll. I really don't know that much about him.