iPhone 4 was the apex of phone design, I will die on this hill!
Unfortunately HP played musical CEOs (three in less than a year) and one of them didn't see phones or PCs as businesses they should be in.
You're also stuck in either portrait or landscapr mode. Whereas a touchscreen smartphone can reuse the touchscreen for say watch a movie or use different input (like say gamepad).
I believe the current innovation trend lies in those small foldables. Of course, small form factor and dual screen has disadvantages for battery, repairability, and ridiculously high price.
The Psion would fit well into a coat or jacket pocket, or cargo pants/shorts, and easily fit into a satchel or messenger bag.
The Psion S3 dimensions are 165 mm x 85 mm x 22 mm (6.50 in x 3.35 in x 0.87 in). Contrast the iPhone 15 at 147.6 mm x 71.6 mm x 7.8mm (5.81" x 2.82" x 0.31"), and the iPhone 15+ at 160.9 mm x 77.8 mm x 7.8 mm (6.33" x 3.06" x 0.31"). Other than thickness, these are pretty comparable.
What it offers most specifically, however, is a useful and usable set of capabilities, in both hardware and software: full keyboard, email, calendar, Web, word processing, spreadsheet, contacts.
I've known journalists who've used it as their mobile platform for gathering and filing news stories, which also means it passes the "professional use" test.
Dimensions from <https://en.wikipedia.org/wiki/Psion_Series_3> and <https://www.apple.com/iphone-15/specs/>.