I've thought about this too, and come to many of the same usability and concerns that you have as well. And to some extent, I do tend to use my domain as a unique ID... though not to the givenname.familyname.countrycode example that you've given. Its made sense to me, and if I had a few more controls on my end to navigate some aspects of communication, I'd have something pretty versatile that could be done, for example:
- visit contact.antoinerjwright in a browser and you get my website, but type it into a messaging app and it comes to me as an IM/SMS/MMS type of message
- a company wants to query me for skills, so they go to resume.antoinerjwright and I get a prompt to the device/service of my choosing that someone is asking for a preview of my credentials, I then offer them some kind of constrained access to this with allowances to how many levels they can share/use it after that

It makes sense, and largely means that we would need to rethink a lot about securuty, access, administration, and even the idea of a file. But, if we went this route, a domain as ID, we'd have one amazing neural map of how we connect to one another, and redefine what regional boundaries look like to the extent that latitude and longitude did to the age of colonization.