There is no doubt that the 764 is a disappointment for Boeing, airlines just do not seem to be interested in the type; the A332 has been very difficult competition for the 764 (even as Boeing supporter, I must admit that the A332 does beat out the 764 in range, flexibility and cargo capability) and many airlines simply do not need an aircraft to cover the small niche between the 763 and 772. Has Boeing made money on the 764 project? Unlikely with the small number of aircraft sold, but the 757/767 lines have been hugely successful for Boeing overall.
The future of the 764 is a difficult one to predict. Its possible that DL and CO will add to their fleets (CO especially seems quite happy with the type); Boeing will certainly try hard to sell the 764 to American....it would be an ideal aircraft to eventually replace the A300s and B762s on routes to Latin America out of AA's huge MIA hub as the 777 is really too much airplane for flights out of MIA and JFK to SJU and from MIA to cities in Peru, Colombia, etc. But that potential order is still a few years off and AA does really like its new 777s so its hard to say. The 764 could also pick some orders up from charter carriers in the US and Europe who are very focused on seat per mile costs (where the 764 does do very well) and we could see one or more of the Japanese carriers going for the 764 for their intra-Japan and regional routes (say JAL if it works out its deal for JDA?).
When the world economy improves, and pax numbers rebound, the 764 will have a "second chance", until then, I do not expect to hear much.