Re: SAPGUI really stank
Moving from the mainframe technology of R/2 DynPro to R/3 kept the business application safe while the runtime looked after the different presentation layers (3270,GUI). So that made sense for SAP standard and customer bespoke application code.
The various attempts to become web enabled; WebDynpro, BSP, ICM, Mobile, showed that a better presentation layer was needed for a multi-channel user experience that was app simple and business process focused. Fiori is that presentation layer and started to give focus to the user experience from Business Suite (ECC 6) onwards.
Innovation with Fiori 3 design has enabled SAP to integrate the UX of all their historically diverse products into one, which returns to the appearance of a single application, whilst really being scaled across multiple cloud instances.
Underneath Fiori the OData services move to a service oriented architecture to better support working at scale in the cloud.
Of course Fiori is the standard presentation in S/4 working on premise and in the cloud. So both SAP Intelligent Suite and customer extensions are safe investments or operational expenses as you choose.