Everything has been saved above: your CPU will consume so much more power than the GPU that the saving in programming time isn't worth the loss of performance/power consumption. From what I measured long ago, anything above 800x600 and your CPU will be spending too many cycles it could use to do something more useful, or just save power.
Even if you need to implement a hardware renderer several times, all you need for a GUI-application is being able to draw some colored and textured quads, it's not like you have to rewrite Unreal Engine 4's renderer and port it to several platforms. And if you care so little about performance that you were considering a software renderer, writing an naive unoptimized hardware renderer will still outperform it.
Abner made the same point using nuklear; I'm more familiar with Dear ImGui
and its samples implement just enough of the graphics API to be able to display any GUI component, you can use them as a starting point for your GUI application without modifying the renderer code!
What Casey did in Handmade Hero's first renderer has value in that it shows us that computers nowadays are vastly overpowered, compared to the performance the software we use can make us believe. That doesn't mean shipping an application with a 16-thread software renderer is a sensible idea!
Regarding your last post, these points don't seem related to comparing software- and hardware-rendered GUI applications, but just that some GUI libraries are closed-source or too bloated to be modified?