I know there have been a few changes to German declensional paradigms over the centuries, sure, but the rest of the Germanic family has pretty much given up on them entirely, or nearly so. In Modern English, the only really conspicuous survival of the system is in the personal pronouns. Everywhere else — articles, numbers, adjectives, and of course, nouns — they’ve been swept almost completely into the dustbin of history. This is pretty universally true of the Swedish, Danish, Norwegian, Frisian, Dutch (though Dutch held on perhaps longest of all of them). Of the Germanic family*, it’s really only Modern (High) German that has stubbornly retained its original declensional system, and the system in use today is almost the same system in Middle High German, and it’s easily recognized even in Old High German texts more than a thousand years old. Why?
Just to give you an idea of the similarities, take a look at the following table representing the definite article in Old High German, Modern German, Old English, and Modern English. As you’ll see, the ancient forms are readily recognized, and it’s very clear that the Old English forms are close cognates to those in Old High German and even Modern German. This gives German speakers some advantage over English speakers when each attempts to learn Old English. Why have these distinctions survived in German, when they have been mostly abandoned by the rest of the family? (Note: I omitted the plural forms from the OHG paradigm because these have, in fact, changed a good deal, with distinct forms for each gender collapsing into a single plural form for each case in the modern language.)
* Modern Icelandic is the other notable exception. It’s still a highly inflected language, but in this case (no pun intended ;), an insular history explains how the grammar has been preserved, virtually unchanged, since the days of Snorri Sturluson. But German is its antithesis: spoken in the middle of a busy continent, by more than one hundred million (compared to less than half a million for Icelandic). German has also been widely used as a language of science, philology, literature, and even music. One should have expected substantial erosion. Why hasn’t this occurred? Any theories?