And even ignoring that, "English was like this" is no reason for it to continue to be like that if the alternative is perfectly understandable. Languages change all the time for all kinds of reasons; we don't use the complex system of verb tenses from Old English, or hither, thither and wither, or yon and yonder, (etc.), so why should we feel obligated to use its pronouns? (side note: which were not the same as modern English pronouns; "you" used to be a second person plural object only, it was thee and thou singular and ye and you plural.)

But yes, "man" used to be gender neutral, and for most of the history of English "they" was the gender neutral third person singular.