The difference between plain English and “high” English is roughly similar to the difference between Bahá’u’lláh’s language, and that of the average Arab or Persian. For whatever reason, Bahá’u’lláh chose to write in a lofty form of expression, and Shoghi Effendi reflected this decision by using a similar stylistic distinction in the English translation. If you ask a Persian or Arab, they will tell you that Bahá’u’lláh is very difficult to read in the original, and that sometimes His grammatical choices are not obvious. I remember a Lebanese man once who criticized a sentence in the Aqdas because its structure seemed confusing to him.
So do we wrestle with understanding God’s Will (the story of life on Earth!), or do we seek to convert it to our preference? I don’t know an easy answer to give the would-be critic. A person either loves Truth in whatever form, or they’re looking for something else.
Perhaps that alone is the reason: To discover who will strive to go beyond the words.
But at any rate, I do not think Shoghi Effendi’s rendering would be needlessly archaic. I presume he intended something by it related to the character of the original text, given that he was an interpretor and not a revelator.