Rear derailleur lifespans
I cannot comment on derailleurs in general, just my own experience. 

My retro ride is 35 years old, and in that time I've ridden it about 150,000 km. It has Campagnolo Record derailleurs, and both are fine. Shifting is still good with the downtube shifters. The parallelogram is rock solid. 

I cannot claim to have maintained it especially well. It's only had about 10 chains and clusters in that time. 

Observing the milage of others, I think the main end of life event is a crash. Some people do seem to wear the jockey wheels, perhaps due to running lower quality chains. 

My conclusion is that it all depends on the initial quality. We get what we pay for.