I just received some sad news from one of my Japanese friends, who was translating an episode. Apparently, in the Bye-Bye Buterfree episode (the Japanese version), Brock tells Ash that once Butterfree breed, they die. That is why Ash was extra sad in the episode. For the English version, they had to change around the script to make it more dramatic and to mask the fact that it was going to die, since Ash was really crying because he knew he would never see it again.
I was really hoping it would come back one day, but now we know that it won't since it is dead. :(