Worth noting that 'Limited Lifetime Warranty' means just that and it is the manufacturer's decision to replace, repair or negotiate as they see fit.
Like as not they are completely unaware of your situation as a result of one of their dealerships making an arbitrary decision based on ignorance, stupidity and *Nothing Else*. It fucks me off no end that arsewipes who don't know one end of bike from the other and have zero practical knowledge can (and do) run bicycle shops when they're not capable of running a bath. Find someone who knows and tell me I'm wrong.
I am no engineer but I do know a thing or two about automotive and aeronautical/aerospace engineering. One of the plus points of rivnuts is that they CAN be replaced. I'd venture to suggest that without this feature any riveted construction, all-metal aeroplane would be scrap as soon as the interior fittings required upgrading or replacement. Can you imagine having to scrap a Spitfire because the pilot's seat started to creak? Fucking ridiculous, frankly.
Your LBS don't know shit from shinola and with what they're trying to shovel at you they should be in the organic fertilizer trade, not bikes. I'll stand by what I said upthread. I've repaired rivnuts, I've replaced them. If you can be arsed you could even get the scrap ones out of the frame next time you take the bottom bracket out but a good dose of Waxoyl or similar into the frame before fitting the new inserts will ensure they don't rattle.