For the first part, you don't need a bike with longer chainstays, but you might need a longer rack that works with your panniers. Although I have an LHT with the long chainstays, I still had problems with heel strike with my old trek rack. Now, the new bontrager backrack deluxe I have allows me to put the load waaaayy back. The Jandd Expedition rack also has a long top rails to allow the panniers to sit way back but it's a pricier rack than the bontrager I have.

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Now, if you're just looking for someone to tell you you need to get a bike with longer chainstays because you want an excuse to buy a new bike: you need a new bike with longer chainstays!