To me, "lifetime" means the lifetime of the original owner, unless specified otherwise. If there is to be a product lifetime, that needs to be spelled out in a specified time: 2, 10, 20 years. There could certainly be disclaimers as to abuse vs material defects.
If the product shows no no unusual wear and has simply failed in storage, how do you determine the "useful service life" of the product without stating it? With luggage, I'm sure most spends far more time in storage than use. There's no odometer, but I think it would be pretty obvious to spot one used by a road warrior flying weekly vs the typical twice a year traveler. Likewise a through hiker vs the summer weekend hiker.
IMHO, the failure of fabric coatings and seam taping is a clear failure of the product to perform as manufactured. As with my example with Eagle Creek, I'm sure they have had a wave of claims due to interior coating degradation, but that is as much a product failure as a seam coming apart or a wheel falling off in the abscence of abuse. It is simply a company policy to reject such claims unless the customer escalates. The real issue is between the fabric manufacturer and Eagle Creek. Not my problem.
Over the years, I have made claims regarding product failures with Outdoor Research, GoLite, REI, Timbuk2, and Pacsafe with stellar, no-questions-asked service. Kudos to those organizations for walking the talk.
As far as REI's purchasing power, I'm sure part of their negotiations includes no warranty returns and they handle it all in house. I'll bet that their gear garage sales recoup a good percentage of the actual cost too.