Here at AUD has taken a big hit since this time last year, not that I'd buy an iPhone outright. I was considering upgrading my 5s but it's working fine, still got good battery life so might wait 12 months, change to a lower plan and save a bit of cash in the mean time.
My 3Gs lasted me about 3 years until I got the 5s so I think I can wait this upgrade out and get the iPhone 7 in 12 months time!
If you really want that 5s to last another year, consider getting the battery replaced. Even though you say battery is fine, you will still notice a nice difference at this point. Though I'd wait and see how it handles on iOS 9 first. But still I recommend replacing the battery after two years. Keep the receipt and show a future buyer that it has a replaced battery and you might get a higher resale price. I think you will find it worth the hassle and cost if you are going to stretch for that third year.