At this juncture, people would choose the 5s because of cost or because they prefer the smaller form factor. There's really no other reason to choose it over the 6 or 6 Plus. The 6 and 6 Plus have better performance, larger screens, higher resolution, better battery life, more up-to-date wi-fi, plus Apple Pay and more advanced motion tracking.
That said, the 5s still holds its own in the marketplace and does not give up much compared to the newer phones. The A7 processor vaulted Apple way ahead of the pack, and remains a formidable performer today. Because the A8's performance gains focused primarily on power consumption reductions (necessary to support the larger screen and higher resolution, while increasing battery life), the 5s could well remain viable and well supported by developers for at least as long as the 6/6 Plus.
That said, the 5s still holds its own in the marketplace and does not give up much compared to the newer phones. The A7 processor vaulted Apple way ahead of the pack, and remains a formidable performer today. Because the A8's performance gains focused primarily on power consumption reductions (necessary to support the larger screen and higher resolution, while increasing battery life), the 5s could well remain viable and well supported by developers for at least as long as the 6/6 Plus.