To be honest, I’ve been waiting a while for something like this. I’ve always wanted something that could be put at the door so people can see who’s ringing the doorbell without having to have their phone.
But people tend to
have their phone
wherever they are in that house (or out of it)... while a screen affixed at the door means they better be in that one room to see who's ringing. If they make some journey to "the door" to see who's ringing, why not just take another step or two to look through a peephole or open the door and greet whoever it is?
I'm guessing you actually mean a BOTH option because there's nothing new about "who's at the door?" I've seen commercials running for many years where presumably Mom or Dad at work can see who is at the door and communicate with that person and/or unlock the door for them. Here's one from
NINE years ago...
Something affixed to a wall at home only works while user is at home... and that user can use the organic interface of hands to unlock a door and greet the "ringer" in a full 3D experience. The one great benefit of affixed tech is in multi-person households because mobile devices like phones tend to leave the home with their owner.
Rationalizations for this are reminding me of first rationalizations for Watch. Suddenly people were coming up with situations where checking a new text on the phone in their pocket was some great challenge that Watch could resolve: "I'm rock climbing and it's dangerous to let go to check my phone... but Watch would make it possible for me to see the cat video while still safely hanging on. So I need a Watch."

How often are you in such rare situations? And couldn't the cat video wait until you are not?
Hopefully there is a very tangible
new idea(s) in this thing. Else, dust off one of possibly many retired iPads in our drawers and put them back into service with dirt cheap wall or tabletop swivel brackets for sale on Amazon like...
There's lots of them in every form factor.
My guess: even though the UI would run just fine on iDevices, it will be locked to only this device so Apple can make their $499+ instead of someone buying a bracket for $20 and repurposing Apple tech lying in a drawer.
We should keep an open mind for an unreleased product... but there
DOES seem to be some hurdles in this rumor vs. just using a backup iPad already in one's possession.