It totally depends on the implementors, if no one implements it, it won't come. If nobody does the first implementation, no other vendor will follow - it's as easy as this.

For WebKit/SVG, features as z-index are easy to implement in the layer based SVG engine - it's currently disabled, but only because it's lacking dedicated tests: I'd still need to write new reftests covering that.

I remember a article already quite some years ago, expressing much frustration with that. Basically the spec is complicated - and the vendors were unwilling to integrate more complexy. And that seemed to not have changed. Most parties are just interested in making things faster and more stable. (And then one bright day, implement a better, cleaner designed 2D vector standard)