[stephenr]

Over time the resources required to run the OS itself should go up slower than the hardware has increased in capability in the last few years.

While the next iOS release may require an iPhone 6 or 6S, I expect it will be quite a while before it changes base requirements again after that.

[kylnew]

The only thing that the iPhone 6 can't do which I know of is ARKit, which seems to be one of the most hardware intensive things phones are capable.