You could fit a 5" screen on the SE form factor if it was edge-to-edge like the iPhone X, or you could keep the old SE 4" screen, make that edge-to-edge, and make the phone itself even smaller.
Illustrated: https://i.imgur.com/OKZiWrN.png
Both of these would be really nice, but I doubt either of them is going to happen.
[0] https://d3nevzfk7ii3be.cloudfront.net/igi/2oa2QHEoq4Ke16su.h...
They did this by creating a new device based on combining a discontinued body and screen with a recent motherboard.
A refurbished 8 would not have the A13 SOC and 3 Gigs of RAM that Ming-Chi Kuo reports the new iteration of the SE will have.
They did this for the original by reusing the already paid off manufacturing equipment from the iPhone 5s line and updating the motherboard and camera.
Eventually, that iPhone 5s manufacturing equipment wears out.
This was the original press release: https://www.apple.com/newsroom/2016/03/21Apple-Introduces-iP...
It's all about the 4" display
>iPhone SE takes a beloved iPhone design and reinvents it
The newer iPhone SE will also take an existing design and update the motherboard and SOC.
If Ming-Chi Kuo's report is as accurate as it usually is, we're looking at a $400 device with an iPhone 8 body and iPhone 11 guts.
On the other hand, hrktb is looking at it from a consumer & marketing standpoint. "What is the unique selling point of this phone? Why should I buy this phone over a different iPhone?"
The selling point was, and remains, the 4" screen. People's hands haven't grown in size in the past 4 years. They still want a 4" phone, because it's still a very sensible size.
For $400 you get a device with an exceptionally high performance SOC that should get five years of of software and security updates.
To put it in perspective, the iPhone SE was announced in 2016, after the iPhone 6 (and ends up being the same generation as the 6S). The 4" is in the title because the new generation were all bigger screens.
They didn't take any random existing design, but the "beloved" 5S, and the cost wasn't that different from a iPhone 6 for instance, which would boast better performance in almost any metrics (short of battery life perhaps). Just to point out the SE resonated with a very specific niche.