My cheapo Giant Cypress (7 spd derailler) has about 9500 miles on it so far, and all I've done with the deraillers is to power wash them when they're crudded up and squirt some lube on them every few times I lube the chain. I've not had to adjust them yet and they're still shifting correctly. They do undershift a little when it's really cold out (< 15*F) but it doesn't really bother me, it still shifts the cables are just sluggish. I don't know that an internal would be less sluggish since I think it's the cables causing it.

FWIW I go through a chain every 2000 miles or so, so that's about 2.5 a year. I've tried different lubing etc but the reality is that I ride 8 miles of gravel a day, and if there's any moisture at all, the chain is coated with fine sand after every ride, and it grinds up chains. I'd have to wash the chain twice a day and even then I don't think it would stop it.