Actually it seems like there have been plenty of contenders with weak defensive 1b. The Yankees with Jason Giambi leaps to mind. Giambi fits the mold of a player who washed out defensively from other positions (2B and 3B) and settled in at 1B. Feddie Freeman had an uncharacteristic bad season at 1B for the NL East champion Braves.
Moreover, many players who were defensive disasters at other positions seem to settle in and become good 1B, and very quickly. Mark Teixeira was a horrific 3B when he came up, but he was a Gold Glove-caliber 1B. Mike Napoli is another. Although it is possible his reputation for being a defensive liability behind the plate stemmed from Mike Scioscia's attitude towards the position. But even if he wasn't as bad as advertised, he was far from an elite catcher. He WAS an elite 1B, however.
Heck, the namesake for this thread Eric Thames is an outfielder who washed out defensively, moved to 1B, and continued poor defensive play for a team that made the post-season this year and lost in the NLCS last year....