Powder Coat is not a magic metal covering. Nothing stands up to road salt for long, especially if it is not cleaned constantly. Springs flex, and powder coat will not stay flexy. Shocks corrode. The only way to protect a shock seal is to boot it.
It seems to me that RE did exactly what I would expect them to, extend the warranty to cover your one broken spring. That's all they were liable for under the warranty anyway. Not your labor, not your tires, not your... whatever.
But wait... You don't want to use their product anymore...
I worked in retail customer service for 20 years. I know your type.
"I'll NEVER shop in your store again!!" The next day you see them there looking at appliances.
Ron