One of the reasons SAP is successful is because it has a large ecosystem of partners adapting and building modules on top of SAP.
Maybe a lot of SAP employees are using SAPUI5, but not that many external developers are, and that's a problem for SAP with S4/HANA, getting them to move from a heavy typed language like ABAP to JavaScript.
If the cost of developing for SAP becomes too high, and training for customer resources too great, it will just be another reason to switch away from SAP. Cost are already too high, even in the ABAP world, and getting Java developers to embrace SAP didn't work in the past. Enterprise development is not the same as web design.