It's a terrible input mechanism for general usage. It's a fantastic input mechanism when you don't have another one handy, open and ready to use. E.g. I use Alexa to trigger my Harmony hub to turn on the TV and switch to the right input with one command, instead of figuring where the hell someone left the remote or digging my phone out and open and app. It's faster, and easier. If a phone call comes in, I don't need to find the remote or the app to mute or pause what's on TV, I just ask for it and it happens. If my hands are full I can still turn the light that I forgot about off on my way to bed.

The "failure" the first time around was that people wanted it to be the main interface, instead of accepting that where it works is as an ubiquitous auxiliary interface that's available everywhere in the house, rather than trying to compete with a keyboard when you're right in front of said keyboard.

Voice competes by working at a distance, even when your hands are not free.

It will never replace a keyboard, as there are so many instances where speaking commands is impractical, but it doesn't need to replace a keyboard to be useful.