The A7 processor inside the 5s was huge leap in performance, and it marked the transition to a 64-bit platform. The A8 focused on reducing energy consumption, which was a necessary trade-off because of the larger screen and higher graphics resolution.
Because both the A7 and A8 only package 1 GB of RAM, and the A8 represents only about a 20% increase in processing power, I would expect the A7 to remain viable for about as long as the A8.
As with other iOS updates, I think you're safe with the first major OS upgrade. But, you'll need to tread cautiously for any major OS upgrades afterwards, and don't make the leap until Apple goes through multiple iterations of bug fixes and optimizations. With both iOS 7 and iOS 8, older devices slowed down much more noticeably in the early releases, and took multiple updates before the speed improved.
On my 5s, upgrading to iOS 8 was a no brainer, and I upgraded on the first day. It added some new features and with version 8.1.1, the speed and stability are now on par with iOS 7.1.2. With iOS 9, I'll probably wait and see first, and maybe wait for a minor interval update or two before making the upgrade.