any well made spa of such a young age will never have any shell problems - your warranty on spa shell and surface warranty period if you are the original purchaser is 7 years.
http://www.hotspring.com/Spas_Built_Last/warranty.html
i have a 95 Sundance Marin and would be disgusted if the shell failed, and even more so if Sundance told me that it was normal - what a load of crap from HS ! If this is their official lifespan expectation, I would strongly warn everyone off purchasing from them - all other manufacturers i have dealt with would never suggest its normal.
I'd certainy demand a replacement spa (warranty or not) if you are the original owner - that attitude is disgraceful.