Anyways, the expectation isn't that Apple build a phone that can survive tight jeans, but a $900 phone should be reasonably durable. A thin flat chassis of thin aluminum may look pretty, but it's not the best choice for durability. All Apple had to do was curve the back to make the entire phone stiffer. A curved back is easier to hold as well, and provides more room for battery - a win/win design choice.
I agree, and curved would be less likely to slip from the hand. They've done before, they should do it again.
I may hold onto my 4S a little longer. It performs just fine with iOS 8 and doesn't have a problem in my pocket. Hopefully they'll address this with the 6S. I always go for S models anyway.