Well, that was sort of my point. If you follow the link to the ROUGOL talk on its history, it explains some of the reasons why.
It's important to consider that the history of RISC OS has an important commonality with the history of AmigaOS: in both cases, the companies were planning something far more sophisticated and clever -- CAOS for Commodore, ARX for Acorn -- but the projects over-ran and under-delivered. So, they fell back on almost-skunkworks projects that used existing tech to get something good enough out the door when the hardware was ready to ship.
In Commodore's case, they used the existing TRIPOS OS, written in BCPL: https://en.wikipedia.org/wiki/TRIPOS ... and the existing Rexx language: https://en.wikipedia.org/wiki/Rexx
The result was AmigaDOS (as it was originally called, before it was renamed to AmigaOS).
In Acorn's case, there had already been a 16-bit port of the BBC Micro MOS and BASIC, for the ill-fated Acorn Communicator, based on the same 65C816 CPU as the Apple ][GS: http://chrisacorns.computinghistory.org.uk/Computers/Communi...
So Acorn ported its existing OS and language to ARM, and wrote a fairly basic desktop GUI in BASIC. The result was Arthur: https://everything2.com/title/Arthur+OS
Once it had shipped, it was finished and renamed RISC OS. Allegedly the name change is because of the Dudley Moore film which had just taken the name "Arthur 2": https://en.wikipedia.org/wiki/Arthur_2:_On_the_Rocks
So, yes, it was a hasty, last-minute exercise, based on an outdated core, an early-1980s, single-user, single-tasking command-line OS for a low-end machine -- just like Windows 3 and DOS.
On the other hand, it worked and worked very well -- arguably far better than Windows 3, I'd say.