2005 Toyota 4Runner Alternator Failure

Battery discharge light came up on the panel and there was a whirring noise when the engine was idling. Dealer assessment was that the alternator was failing and had to be replaced. Root cause was salt build up, because salt is used on the roads as de-icer during the winter. A plate was installed on the new alternator to block the are where salt builds up. Does the reasoning make sense? I had the 40K maintenance done 3 weeks prior to failure - shouldn't preventive maintenance have identified the potential faiilure? Seems like fraud - the vehicle is not built to be driven in snowy winter states or shouldn't be sold where winters are harsh if alternator will fail so soon due to salting of the roads.