sometimes stuff just goes out. My ranger is a 93 with 165,000 miles, and my master cylinder just went bad. But a "mushy" peddle isn't necessarily a bad master cylinder. You could have a leak in your brake line, air in the line, or just bad brake pads, bad calipers(or goin bad)... there's a list of stuff. Now if you lose brake pedal pressure while you have your foot down on the pedal, i'd definitely change it quick haha(thats what mine was doin). I actually had to set the parking brake at lights in town while i had to run and get the part.
As for your brake wear, it depends on the quality of brakes used, and what kind of a driver you are. and needless to say, i have a tough time believing everything a dealership tells me, so it might be a good idea for you to check everything yourself, and if you don't know enough to do that, get educated. people don't screw ya when you know what you're talking about... i.e. a tire place wanted to replace a lower ball joint in my truck for a $150, well i went and bough the part, got a press and spent $15 for the ball joint and did it myself. Just to give ya an example.
93 ranger ext. cab 4.0L auto 6" SJ/custom lift--37" super swamper truxs M/T tires--lock rite locker w/ 4.56 gears up front--warn hubs--exploder 8.8 w/ disc brakes 4.56 gears and detriot locker--one-piece driveline--bushwacker cut-out flares--Bilstein 5150 shocks--2 10" xcite subs w/ 1600W Crunch amp-- tinted windows
waiting for: dual steering stabilizer, put on 3" body lift(in my room), dual shock setup, electric fan, possible SAS with a waggy D44