with surpantine belts generally can last up to 100000 miles. Sad that this happened. I wonder who else they have taken advantage of and stolen from?
Serpentine belts today are made of EPDM (ethylene propylene diene monomer) and these types of belts don't typically crack when they are worn out like older belts of about 10 years ago. The grooves will widen and get deeper eventually, the grooves of the pulleys will bottom out on the grooves of the belt, and then the belt will start to slip. According to one belt supplier, as little as 5% of rib material loss and surface wear can affect how the belt performs, and just 10% belt slippage can affect the overall drivability of a vehicle.
Belt manufacturers have simple gauges that can measure groove depth, overall thickness and cracks to quantify wear. I showed you the gauge and the gauge showed the grooves in belt were to deep, and on the flat / back side of the belt you can see the lines from the pulley ribs due to the grooves were to deep. So according to the belt manufacture the belt is worn and the belt manufacture recommends it to be replaced. The napa belt was not brand new. You also mentioned you were at the dealer ship and they also had mentioned the belt was worn.