This makes complete sense, except it doesn't take into account who we're dealing with here.

CO management is extremely arrogant and is not accustomed to competition. I think it took them this long to realize that people have left and they will have to work to earn back that business (hence the recent changes). Recall Jeff's comment about the road being closed, and now the road is open again, so people will come back [on their own]. Prior to that they were in denial that customers had left, which they now freely admit. They have no idea what they're doing in terms of managing their customer relationships, and they completely discounted what is important to the lucrative PMUA customers. But they cannot see that because they really don't understand it (let alone, their culture discourages them from admitting another carrier does/did something better).

So I would not necessarily attribute CO's lack of a significant competitive response as failure by AA and DL to poach effectively. Rather, I would attribute CO's lack of significant response to their naivety and unfamiliarity doing business in a much more competitive environment that they now find themselves in.