Like any loaded question that cannot be answered immediately, only time can tell.
Undoubtedly, from a contract standpoint, it is hard to know if the Twins made a mistake not dealing Morneau.
Yes. He is underperforming for someone who gets paid $14 million a year (.271/.351/.472, on pace for 21 HR and 80 RBI). However, who honestly would want to pick up that tab from the Twins? Especially when Morneau's recent concussion issues are considered. Plus Morneau's contract is up in 2014. So if the Twins can handle Morneau for one more season, they can cut ties much easier, but will shell out an additional $14 million in 2013.
On the other hand, if they would have dealt Morneau, they would have received some money for him. Some is better than none. Perhaps a playoff-bound team in need of some pop to their lineup would have been willing to make a deal with the Twins.
No such luck.
The Twins are stuck for better or worse, for the rest of the 2012 season and mostly likely 2013.
Who knows? This debate might be irrelevant at the conclusion of the season. Morneau might end up hitting .300 and hitting 30 HR and 100 RBIs. He could also get injured and have season ending surgery. Hopefully, Morneau finishes 2012 with the former.
The point is: no one can truly know if the Twins made a mistake at this point. Only at the end up season will people be able to completely see if it was a mistake or not.