I bought a brand new bike last May (2015 Brodie Elan) and proceeded to put 11,000km on it by November (cross Canada ride plus some commuting).
Around the 11,000km mark I brought it back to the shop and they checked the chain wear, using some sort of spring-loaded Park Tool, which indicated that the chain was totally dead as well as the cassette.
I'm not clear if the chain wear gauge directly indicated dead casette, or if it was more "if the chain is this worn the casette is definitely dead too". Either way this did not surprise me as I had been experiencing some gear/ tooth hopping that I could not correct with cable tension adjustments. I asked them to replace both casette and chain.
After getting the chain and casette off they found the bottom bracket was also dead. This also was not surprising as I had been experiencing some "gravely" feel while pedalling.
So they replaced chain, BB, and casette. However, they said the derailleurs and chainrings still had life in them and did not need replacement.
I'm wondering if that makes sense, i.e. would derailleurs and chainrings be expected to long outlast chains and casettes with which they were purchased? Or should I be worried that those could be in a heavily-worn-but-not-quite-dead state that in turn would be wearing out my new chain and casette faster? How would I check?
For reference, here is what was originally on the bike when new:
CRANKS: Shimano Deore 26/36/48T CASSETTE: Shimano Deore 11-30t FRONT DERAILLEUR: Shimano Deore REAR DERAILLEUR: Shimano Tiagra BOTTOM BRACKET: Not published but I'm guessing low end Shimano