Part of the reason why single-letter variable names persist may be due to multiplication. There is no good symbol for multiplication; the two most common ones,

Part of the reason why single-letter variable names persist may be due to multiplication. There is no good symbol for multiplication; the two most common ones, × and ∙ are easily mistaken for other symbols (x and . respectively) when improperly typeset or handwritten (the distinction is easier with proper typography and neater handwriting, of course). The way most mathematicians ignore this problem is by throwing two variable names next to each other, so xy is x times y. Obviously, this system is incompatible with multiple-letter variable names. I have unfortunately seen the two concepts mixed, which makes reading an equation like reading ancient Greek or Latin (which lacked spaces and punctuation). Until there is a commonly accepted, easily distinguishable symbol for multiplication, I don't see single-letter variables going anywhere, or multiple-letter variables being used in general mathematics.