Alice, don't let them replace the chain and sprockets needlessly - if there is still plenty of 'adjustment' left then they should be fine. I didn't get mine done at 18,000, though I might at 24,000 (coming soon). However it does depend on the individual dealer's view on what *has* to be done for warranty purposes.
The rollers will need checking though - mine are well worn after every 6,000 miles (Dr Pulley sliders going in next) - and the original wait until 12,000 miles for them to be changed found two of them worn down to the metal insert and the rest very close.