If you’re talking about 5-year-old Apple products, the first Apple Watch is also 5 years old and runs at 520 MHz with 512 MB of RAM. I expect it too is usable. But that’s quite a bit different from 64 MHz and 64 KB of RAM.
I wouldn’t be too sure of that. It was slow out of the gate and got progressively worse as time went on; it’s entirely possible that a much slower device might feel more responsive.
That's a kind of a… difficult comparison. The 6S was right when Apple shifted from chips that were highly competitive to chips that were ridiculously overpowered.
If you're making a point about 5-year-old hardware being still useful today, I think you need to reference typical 5-year-old hardware. The 6S was weird (albeit in a good way).
My iPhone 6S is more than 5 years old, and is perfectly usable as a daily driver (and my SO's iPhone 6 is too - it's 6 years old, only runs iOS 12, but still gets security updates and is perfectly usable).
It's fine if on the freedom/performance/up-to-dateness tradeoff, you're not pine64's customer.
Many people are.