I don't see how an i7 would stay relevant longer than an i5 since the performance difference is rather minimal if you're comparing within the same generation though...
The number of cores will have a heavier impact on future usage?
Both an i5 and an i7 have four cores. However the i7 has hyperthreading when the i5 does not. Still, ultimately the performance difference is small.
I'm referring to general tasks, not just gaming.