Think about it, we start off with lyrics about how cowboy life is supposed to be freeing but is actually corrupting, then we hear about (I assume) Lefty leaving his Momma and heading out, then we get a few lines about what a badass Poncho is. I think it was Steve Earl that points out that at no point are they shown interacting with each other.

Poncho is presumably gunned down by the Federales in the borderlands, but we see that even years later the few gray Federales are telling themselves they totally could have caught Poncho any day.

Meanwhile, Lefty and his mysterious bread are in Ohi-O long enough to start growing old. Do Mexican authorities have the resources to offer a big enough reward to keep a man holed up in an American hotel for the rest of his life, even a cheap one?

Lefty can't sing the blues all night long anymore because of the dust that Poncho bit. When has guilt ever STOPPED anyone from singing the blues? Perhaps Lefty can't sing anymore because the dust Pancho bit did not just go to his mouth, but also his lungs?

Alternate theory, Lefty/Poncho working with the Federales faked his death out in the border lands where no one would see, took his share of the reward on Poncho's head AND whatever money he had stolen as Poncho and took off. His bandido lifestyle has left him with emphysema and some regrets about taking the easy way out.

Pancho and Lefty, classic story of betrayal and guilt or classic heist story?