To use just the TLD, the domain will need to be fully qualified with a trailing dot, "x@tld.". Otherwise, the domain can get confused with just the hostname in the local domain (tld.example.com). Just the hostname is a valid email address but wouldn't need to be accepted by most signup forms.

My guess is that nobody will use just the TLD because of the confusion. And because a lot of software does not support fully qualified names.