Apple doesn't push the arrow until the wood breaks. They take the pointy end, aim and ship (RealDevelopers). SteveJobs didn't want the iPhone to merely fix our gaze but service our needs. Siri could break the paradigm and shift its focus off the screen back onto our needs. Steve saw that, that opened up an entire handheld services market and Apple would own the abstract layer between services and customer through Siri.
Apple didn't drop the ball on Siri. Siri hit a threshold, limit or criticality beyond which Five9's reliability eroded. They had to build infrastructure, to host Siri demand. The marketplace would help test and build a Siri strong enough to bridge the paradigm. Its not there yet.
Developers ship and Viv stands at the pinnacle of dreams while Siri is shipping daily.