I don't want to trade Lynch, but I think there are valid responses to your two points.
First, playing well is not a reason to not want to trade him. After all, buy low and sell high is the most basic principle of business.
Second, you might need two backs to succeed in the NFL, but you don't need two backs of the quality of Lynch and Jackson. And if you have a surplus that is being used inefficiently at one position and deficits at so many others, it makes sense to try to reallocate your resources.
BTW, here are the YPC of the top two backs on the four best teams in the NFL:
Minnesota: Peterson 4.5, Taylor 3.8
Indianapolis: Addai 3.7, Brown 4.5
New Orleans: Bell 4.2, Thomas 5.3
San Diego: Tomlinson 3.2, Sproles 3.3
I'm not sure how good any of those backs are, outside of Peterson.