I wouldn't call it a "fundamental engineering problem", but over the past 10 years there have been some pretty insane strides in power consumption, size, screen technology, and software that have made what we consider today to be the "smartphone" possible.

The change in laptops hasn't gone a fraction as far as it has in cell phones in the past decade. That might stop soon, but until it does getting a phone to last 10 years is a pipe dream, unless you are okay with a severely out of date device, which again, I don't think that 99% of the public will be.

And without customers, a company won't be able to support a phone for that long, so you'd end up with a slow, expensive, dated looking, insecure device.