Thanks for the replies everyone . I get around 2500-3000 miles on my chain before it gets stretched and gritty enough that the shifting becomes imprecise and the chain noisy. I tend to replace it at the lower end of Sheldon's 12-link test rule, mainly because I can't stand the noise when I ride. I do lube the chain a lot (perhaps too much), but cleaning is limited to wiping down with a rag which leaves a lot of grit behind, especially around the bushings. I think about converting to an IGH each time the chain or cassette comes up for replacement, but at the last minute, it's easier and less expensive to just replace the worn out parts (although maybe more expensive in the long run).

CCrew, I wouldn't mind switching to a mountain cassette. I have a spare XT rear derailleur that's still in good shape, so other than shifters, chain, and cassette, I think I'm set. I wonder about my FD since Shimano's site says the R773 (what I have) is only compatible with R770 shifters (also what I have), and that the R440 shifters (the 9sp that I'm considering) is only compatible with R440/R443 FD, but both derailleurs are designed for road triples and seem to have the same shape and cable routing, so I think it would be okay.

OTOH, if going 9sp will not significantly extend the life of these parts, or if there's a way to keep fine oily grit from grinding down the 10sp parts so quickly, then I'm happy to stick with 10. The extra gear with the 10 isn't that important to me, though.