Screen size is the only thing that might stop changing.
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
But everything else would change. I'm not saying you're wrong. But I don't see where the "optimum" has been reached?
We will definitely see more RAM and processing power. And I thing the high-performance phones of our day will take a backseat in the future. Apps of the future will be quite powerful and resource-intensive.
At some point, all the low-end phones will be unable to use most of the Play Store too.
With all of that said, you asked an important question. Do we need it? No, not really.
![Frown :( :(](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
We don't need it. I'd rather stick to desktops and laptops.
![Stick out tongue :p :p](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
Easier to upgrade/maintain. Phones are great. But it needs to stop at some point in time.