There is no point. We realised it only recently. If you remember the cell phones from the time before smartphones, there hadn't been much technological progress. My first cellphone, a Nokia, could store up to 10 short messages. It's pedecessor had the same storage capacity. Of course, there were technological milestones that have been passed, e.g. antennas which didn't protrude out of the phone, vibration motors, (in comparison to today) really shitty photo-cameras (and the buggy software that was needed to transfer the photos to the computer), etc.

The point is, that they all were capable to do the same thing: calling and texting. Looking back, there was not really a need to replace the old cellphone. Advertising made us want new shiny things.

This changed when smartphones emerged. Hardware wise, there are not many differences. Some have faster processors than others, others have better cameras. The storage capabilities are sufficient. For the normal user these specifications don't matter. All smartphones are capable of accessing the (real) internet. The main difference today is in the software (operating system). Older phones run on software that is too outdated to keep pace, and the software support is often limited, which as a result leads to possible security flaws - because the user is supposed to upgrade the hardware, not the operating system only. And that's why new phones are bought, despite the old ones would still do.

My smartphone ist running on Android 8 (Nougat). It's still working and is sufficient for my needs. But I wouldn't run my online banking with that phone. Also, it gets pretty hot and slow when navigating with Google Maps.

Conclusion: It's not the hardware specifications which lead to the replacement of smartphones. It's the more complex (security wise) software requirements certain applications (online banking apps, medical apps, e.g. insuline tracking apps, overall more sophisticated apps that runs slow on an outdated smartphone) demand today.