A few weeks ago, my wife and I travelled to Portland, OR to visit her daughter/ my stepdaughter. Since SD was working we decided to help her out by taking her 6 month old bike in for its first tune-up. For the record, the bike is a Specialized gravel bike- entry-level bike, perhaps, but certainly not a WalMart BSO. So I was stunned when the mechanic said it needed a new chain and cassette. Already? Granted, she has been commuting on it regularly (16 mi. round trip?) but still. My (usually suspicious) wife accepted this news but I balked. The mechanic finally offered to charge us for the parts only since he was working on it anyway, but I still can't escape the feeling we got hosed.
By comparison, my wife and I have 6 bikes; 5 of which get ridden regularly, 4 of which are 20+ years old and 3 of which retain their original drivetrains. Even the newer ones don't get serviced anywhere near this often, and while I'm sure we tend to stretch the maintenance intervals, all of them perform well- no gear-jumping or other problems you'd expect from excessive wear or misuse.
So- what are proper intervals for various bike components, based on real- world usage? Should one go by mileage intervals, or by performance declines? And what are the potential pitfalls of being more, shall we say, casual with upkeep?
Thanks!