Is the iPhone 5s still viable? For one year, no longer. As of spring, 2017. In many parts of the world, it is still sold brand new with Apple warranty at bargain prices. Consider that just a month or so before Apple still sold the iPad mini 2 on its site with the same processor, pushing much more pixels.
The main question is, is the 5s noticeably slower than the 6?
According to Geekbench scores, the A8 chip is not that much faster than the A7 (the big jump in iPhone CPU performance is from A8 to A9), and the iPhone 6 has to push more pixels than the 5s.