I think the iPhone 6S is actually the oldest phone to which this applies. It was a big step up in processor speed from the 6, and I think it marked the point at which phone cpus hit "fast enough". Hopefully apple continue to support the 6S for at least a couple more years, because I don't think there's any technical reason why they can't.
If not for the camera and iOS support inevitably ending, I don’t see much reason to upgrade.