This for sure. Grease and oil dry out. This is a certainty. Of equal importance is cable housing function. The stuff degrades and degrades slowly. Most people think their "shifters" are going bad when things lose that nice, light shift feeling. In reality, it's cable housing degradation that imparts more and more friction in the system. Friction grows and the rider simply has to push harder and harder on the shifter just to get the cable to move in the housing. This kills shifters.

Fresh housing (and cable while you're at it) properly prepared and installed makes a huge difference. Replace housing BEFORE friction grows to the point where you're stressing the shifter. This, plus occasional shifter lubrication, can extend shifter life by a very long time. I have 33 year old STI that's still functioning perfectly fine.