Yes Fiori is the next User experience. 
But this doesn't mean that SAP GUI is dead. It can never be dead. SAP UX Strategy is to ease User's headache of going slow, complex UI.
Fiori is simple,coherant, responsive(With the power of SAP UI5). Good point is it can be accessed through any device.
With the launch of S/4 HANA, frontend being Fiori. SAP has already set a benchmark by saying that they are not behind any other ERP tool. Just head back, relax and enjoy new SAP UX.
But ya, enhance yourself with new skills like SAP Netweaver Gateway, SAP UI5, ABAP on HANA
Cheers!!