Arthur to RISC OS 2 wasn't an evolution, it was a fresh start. Almost no commercial software used the primative Arthur BASIC desktop, but implemented their own full screen windowing system, none survived the transition to RISC OS 2, they all needed a rewrite.
What about First Word Plus. I remember back on RISC OS 2 (not around in Arthur days), that used it's own full screen mode, own filer etc. Always thought it seemed a bit strange when compared to other programs. I guess it's because it was written for Arthur.