It's pretty clear that this isn't about Conan vs. Letterman ratings, which are pretty even in key demographics: NBC announced that Conan beat Letterman in the 18-49 demographic, while CBS announced that Letterman beat Conan in the 25-54 demographic.
http://tvbythenumbers.com/2010/01/04...-quarter/37565
http://tvbythenumbers.com/2010/01/04...-winners/37559
In other words, they're pretty close in 25-49 demographic, but Conan has more in the 18-25 demographic, while Dave has more in the 49-54 demographic. If you're NBC, it's not reason enough to pull the plug on the current lineup, especially if you're looking at the long-term investment.
If you look at the 18-54 demographic as being the important one, The trend is that every year, Conan's performance against Dave will improve as young viewers enter the demographic, and older viewers leave the demographic. People don't easily change their allegiances: talk shows provide a comfortable familiarity. The young people who begin to watch Conan now will still be watching him in fifteen years, and there's no reason to think that he won't be the best-rated talk-show host during that time.