Actually my wife and I have been thinking about this sort of thing for years. (Not the light bulb.) We figure that on-screen evidence suggests there are really three Goa'uld genders/castes: females (queens), males (lords, for lack of a better word), and neuter (drones). The males and females comprise the System Lords, and the drones do a lot of the grunt work. Drones probably make up the vast majority of the Goa'uld population.
Queens have been stated to be able to spawn offspring on their own, but logically (and biologically) these would all have to be drones. You can only spawn a lord or breeding male if another lord impregnates the queen (though probably you get a lot of drones that way too). That's why Skaara was presented as Apophis' "son" and was treated as special-- breeding males are rare compared to drones. (It's a little unclear how a lord would impregnate the queen while in a host body. I'm trying to envision back-to-back sex, and failing. However, it has to be something like that.)
It doesn't seem logical that chowing down on a few dozen symbiotes would make a dent in the population, considering the numbers that seem to exist. However, it would make logical sense if the System Lords were just eating young males. Keeps the breeding population down and gets rid of the competition at the same time. They probably wouldn't bother eating drones, because they aren't part of the power structure.
It's probably possible for a drone to amass enough power and followers to become a System Lord, but because they are unable to reproduce and spawn offspring loyal to them, this is likely to be rare. They are probably considered second-class citizens (although still higher than Jaffa). They are more or less expendable, so if one matures and no host is available, they just let the symbiote die. This is probably the answer to the OP's question.
Any thoughts? This would address a lot of discrepancies seen onscreen.