Not thrown away - recycled.
Nobody said anything about changing brakes, filters, or whatever before it is necessary. That's the beauty of SCHEDULED maintenance. If the mechanic KNOWS he will see the car again in 3 months, and the brakes, filters, watever still have at least 3 months life left, they do not need to be changed YET. If they are still good for another 3 months 3 months from now, GOOD. Leave 'em in. When they get to the point they will, in all likelihood need replacement before the next 3 months is up, replace them NOW and save the customer an unscheduled repair. So you are wasting something like one month's brake life - great big hairy deal.
Not everyone can do their own maintenance - that's why a good mechanic who has the good of the customer in mind is such a valuable commodity to the AVERAGE car owner. No flat rate pay schedule in My shop. The mechanics were paid by the hour, so cheating didn't make any sense, or any cents. No commission paid to the mechanic for "upselling " a job either. I paid them a darn good hourly wage - whether they were punched on to a customer's job or sweeping the floor. I got good value out of my dollar, and so did my customers - that's why they kept coming back, and recommending their friends, and buying new cars from the dealership.
Remember, the salesman only sells a customer his FIRST new car - the service department sells them the next one, and the next one - and so on.