The iPhone 4 is the only single core device currently running iOS 7. Very poor choice by Apple. A5 should have been minimum spec to run the latest software. iOS 7 is really not optimized fully as is with the dual core CPUs so there's no way it should be running on something as ancient as the A4.
But to answer your question...the A6 is still very powerful and should be able to run the newest updates well enough for the next couple years. 7.1 is going to be a godsend for those on 7.0.4 and that should fix a lot of the problems and issues people are having.
In a few years you'll be posting the following:
The iPhone 5c is the only 32-bit device currently running on iOS 10. Very poor choice by Apple. A7 should have been minimum spec to run the latest software. iOS 10 is really not optimized fully as is with the 64-bit processors so there's no way it should be running on something as ancient as the A6.
The 5c is only as future proof as the iPhone 5.