My understanding is that you replace the chain and cassette (at the same time, because they "wear together" - though I know a few who insist you can run 2 chains to 1 cassette) early enough to save wear on the more expensive chainrings.

Once the chainrings are worn you replace them too. You can replace them individually on some bikes, not on all. And then you should replace the chain and cassette at the same time.

The upside is you then have a brand-new drivetrain, which is great!