Name server records are not what "defines the domain's very existence", it only defines whether the domain exists in DNS.
There are cases where e.g. name server changes or domain name transfers results in a loss of name server data in the root servers. The domain still exists, but it is or will quickly be in an unusable state.
So, to reiterate:
DNS shows you whether the domain works.
WHOIS currently shows you whether the domain exists, as well as domain ownership information.
If ICANN wants to get rid of whois for domain names, it needs to replace it with something else.
Functional EPP implementations would do fine for those of us who are registrars, but leaves the public with no practical way of yielding ownership information.