The Echo cannot take over for a number of reasons.

1. No guest will now what commands to use
2. You can't browse anything with voice control. Alexa, read me my 5000 sounds and let me pick one.
3. You can't even really get good information beyond the trivial because takes so long to speak it that you'll have forgotten half of it.
4. It can't tell you the state of the house because it'll take too long
5. It can't talk to any serial devices, and presumably no USB devices, right?
6. It's not extensible or customizable unless you use a custom skill, and immediately you run into the problem that you now have to use specialized syntax, Alexa, tell Bubba to do this or that. Most people won't know when to which.
7. It can't do anything securely. Will you ask everyone to leave while you speak your password?
8. It cannot react to anything, it's purely control, AFAIK. It can't even run scheduled activities, can it?

Even a mildly adventurous DIYer will probably quickly move beyond those limitations. And the demonstrable benefits beyond that of even a fairly limited real, local system are enormous.

And of course you can always play the FUD game, and with justification. It won't work when your internet connection is down. So at the time you might most want to get things done quickly, it may not be available to you. And of course the issue of do you want these companies to own your lives?