The iPhone 6's Bendgate was something extraordinary when Apple first released it. I knew friends that had it and weren't thrilled with it when it came out. But they also didn't return the phone either. So if anything, it will just take time. And after that, I feel it will be standard for other phone companies.
The sales will momentarily dip for the iPhone 6 until Apple does one of two things. The first is to revamp the phone with a stronger housing to prevent bending. This probably the least likely at this point. The cost would be staggering to redesign and redistribute the phone. Second option is to create an attachable housing for the iPhone 6 and offer it to the owners for free. This while, costly, would allow owners to keep their phones and reduce the episodes of them bending. Once the new "shells" are sent out, Apple should stop replacing iPhones unless the shell and the phone is bending.
There have been many hiccups in Apple's past when new products are released. It has not proven to hurt sales in the past. People wanted a lighter, thinner IPhone and Apple delivered. Users will know to take care of their phone and there are cases that can be bought to insure the safety of the product. The benefits of the IPhone 6 will outweigh its bending.
I don't think Bendgate will hurt sales of the iPhone 6. Most people understand that a mobile phone is an expensive and fairly easily broken item, and are careful with their phones and what they do with them. The people who manage to bend them are obviously doing something silly with their phones, and most people realise this.