2009 Volkswagen Jetta 2.5

2009 VW Jetta

Dealer recently replaced battery and fuse/relay box. Now they are trying to sell me an alternator. My car has only 55k miles. Service adviser said they performed a load test while replacing the battery. They said alternator is producing excess ripple and will continue to work until diode burns out and car will no longer crank. They said I may have to jump start. Battery voltage 12.41; charging voltage 14.32 ; battery voltage after turning engine off 12.71.
Did AC ripple test at battery and read 0.047-0.051 volts AC or 47-51 mV AC.
The lights are not dimming, never had to jump car, battery light on dash never appeared. Why is the dealer trying to sell me an alternator?