They could have contributed their IndexDB engine to webkit at any time but chose not to.

I would also remind everyone that Google chose to fork Webkit to create Blink. Safari is actually the first to 100% ES6 compliance Chrome sits at 89%. Looking at the list there are plenty of features supported by Safari and not supported by Chrome including Audio Tracks, Video Tracks, ES6 modules, SVG Fonts, Animated PNG, CSS regions.

I don't think it is fair to discount battery life, memory usage, and performance so heavily.