My LBS was surprised/impressed when I said I'm still running my original rear derailleur on my 2009 Kona Dew Drop. I've tracked roughly 15,000 miles on this bike over the years, and those aren't easy miles either... I ride gravel centuries, CX, single-track, commute in all weather .... so this derailleur has see a little bit of everything.
What do you consider the lifespan of a derailleur?
For reference, I believe this is the Shimano Deore 2203:
