I am come has its roots deep in the history of the English language. In Old English (1200 years ago) there were only two tenses, the present tense and the preterite. The present tense was used to express the future as well and the preterite had the meaning of the modern past tense, continuous past, perfect and pluperfect. However, even in those days the periphrastic tenses were sometimes formed, as in Modern English, by
hæbbe and
hæfde with past participles and they sometimes had the meanings of the modern perfect and pluperfect respectively. Usage of tenses wasn't very settled in Old English, and there was even some usage of the future tense even though it didn't officially exist.
Will often indicated volition and
shall obligation, but not always.
Periphrastic tenses of intransitive verbs were formed with
wesan (= to be) instead of
habban (= to have), as in Modern German. This explains why we encounter
is,
are, was and
were as perfect and pluperfect auxiliaries as late as the 19th century. It also explains why it is so easy and natural to say:
He is gone.CB