And in many areas it would've been just as good as what we have today.
Today the problem is a) the voice recognition part, which happens on-line though it shouldn't (introducing large latency and unnecessary Internet dependency), and b) scriptability, which is something none of the voice assistant vendors want to give to you, for business reasons. A better reality would have voice parsed on-device (possibly with models from on-line service providers), and available as a voice-to-text API that could be easily accessed by local scripts.
A friend once said all we need is 'the sinew between the questions'. Connecting them (contexts, intents, etc) and making it close to 1.0 probability on decisions through the entire conversation.
I think the 'marble rock' of Jarvis is there already, and with ML/AI we can likely make it 'David-shaped' but the refinement, 'warmth' and complexity of Jarvis is man-hours that would likely be better spent elsewhere.
Jarvis in the movie was Hollywood smooth: made to look great in a movie. Even the goofs were great, and it behaved like a puppy, to be likable to the audience. You don't need that in a work assistant. But if you remove that, plus the robot-arm, you're left with a voice controlled Outlook assistant, which is useful, but not sexy.