Darn, hit the wrong key; I was actually going to explain my retort
It’s true that nominative plurals of words in the a- and o-declensions are not formed in “-s”; hence feminae, “women”, viri, “men”, saxa, “rocks”. However, oblique plurals often do end in -s; feminis, “women” (ablative), viros, “men” (accusative), saxis, “rocks” (dative). In most modern Romance languages, we find not only a reduction of the rather complex Latin declensive and grammatical structure into the simple masculine/feminine opposition, but also a reduction in the case structure to nominative/genitive (and sometimes an accusative, or generic oblique, case). Unless we are to demand that nominatives in modern day languages inherit only from nominatives in their ancestors, we can’t disallow plurals ending in “-s”.
It’s also true that adjacent languages affect each other, no more so than when the two languages are sitting atop one another. Gothic and Old German have long since vanished. In the first case, Gothic was replaced by Castilian, Catalan, and Provençal; in the latter case, not only by French, but by English, Dutch, High and Low German, and Yiddish.
In the case of English, in was affected, not only by the Norman Conquest, but also by Danish/Norse conquest and settlement, and by the intrusion of Germanic conquerors and settlers into a Celtic milieu. Had the Saxon forebears of the English stayed in Hanover, their dialect might have remained more complex; as it was, creolization reduced it to as simple a form as could be derived from Old German whilst remaining unambiguous.
“Kings die, and leave their crowns to their sons. Shmuel HaKatan took all the treasures in the world, and went away.”