Replacing items such as the fuel filter and spark plugs are part of the routine maintenance schedule. It's not unusual for spark plugs to be changed every 30,000 miles or so, depending on the type of spark plug fitted and how the vehicle has been used. It could be argued that these items are still in good condition and do not need to be changed, but they would require an examination to determine the condition. We would suggest that the purpose of good maintenance is to ensure the vehicle is in the best possible condition, rather than waiting for problems to occur.