On the scales that you’re talking about — fewer than a hundred unique sites put together (15 vs. 69 — a ratio of 1:4½, not 1:6), both forms are so rare as to make such comparisons almost irrelevant. For this hypothetical reader who wouldn’t be able to figure out what ecumenopoleis means from the context, it is given as a plural form at the beginning of the article. It is patronising in the extreme to assume that using the etymologically correct plural will cause such difficulties — particularly as all readers need do if they are confused is open the ecumenopoleis article in a new tab / window to be told what it means in the first sentence. The last remaining argument is æsthetic — it is undeniable that a  ending sounds a lot better, and is far easier to say, than a  ending.