You are right in that without his fastball the CU is nothing. There are many things that make Lincecum special- his deceptive windup and delivery to his small frame and yet large stride.
But the thing propelling him into stardom is his use of offspeed pitches. Specifically, his CU as a pitch to screw batters up is what is making him so great. Almost every pitcher in the big leagues can throw with his velocity but very few can throw a CU with such effectiveness as he can, and that is specifically why he has a low era and is again in line to get the Cy Young award.
I ask- is it his fastaball that is exceptional or is it his CU? It is obviously the CU. That is his best pitch.
I will always argue the best pitch in baseball is a properly thrown CU because it mimicks the properties of the fastball in every way only that batters can't hit it- its like its a cruel magic trick.