Sometimes a form comes down to us in a way that makes it fit right into a entirely different paradigm. In this way, we often easily accept or do not even notice old usages that have persisted. In fact, they persist exactly because we do not notice them. If we did, we would hammer them into the current paradigm.
An example is the phrase “he is gone”. On the surface, this looks like any normal combination of the copular verb with its subject, followed by the past participle of a verb used as a descriptive adjective. Similarly, we say “it is broken”.
However, we don’t use intransitive verbs in this way. We don’t say ’he is left’ or ’I am arrived’. Someone might think of the verb ’to come’ and think: “Well, I have heard ’he is come’, but not recently.” And therein lies the key.
English during the Renaissance period and before had a system of helping verbs similar to that of German and French where with change of state and motion verbs the auxillary verb was not ’to have’ but ’to be’. This usage was disappearing during the Renaissance, but survived long enough to give us forms like “Christ is risen” and “Christ is come” in the Authorized Version or King James Version of the bible. That’s where we still hear ’is come’, from the pulpit.
The only survival is the phrase, ’he is gone’, ’I am gone’, ’they are gone’, etc.