NV200 alternator belt just gone

2016 NV200 with 21k miles and the alternator belt has just gone. I heard flapping under the bonnet when driving and on inspection the belt was starting tom strip.

Gone to main dealer and asked for a replacement under warranty (5 year bumper to bumper Nissan warranty) , which they say is doubtful as it is a wear item but highly unusual.

I think it should be replaced under warranty considering the age, mileage and rarity of the fault. 

If Nissan refuse would it be possible to challenge their decision? 

An aux belt snapping that prematurely I would yes challenge their decision and complain to Nissan.  The Ford interval for Aux belt changes is 100k or 10 years!!  No way should one be snapping that early.