But why doesn't the developer using OpenGL for rendering? It does all the shit work of drivers, and what are the downside of this?

Is Inkscape's output rendered by OpenGL? If isn't, why not? It could improve the program performe a bit. A downside is that a such implementation could be hard to apply over the existing codebase, but it seems that the current rendering engine uses the native cpu instead of hardware. A though

Though there is not any kind of live color feature implemented in inkscape, there is an easy way to do such behaviour in inkscape. Create a new gradient, name it whatever you want and then open the XML editor. Expand the svg:defs element, click on the apporiate gradient you've created and remove one...