The decline of weorðan in English versus the grammaticalization of werden in German

The passive construction consitutes a marked difference between English, which uses the auxiliary be, and German, which uses werden ‘become’. Zieglschmid (1931), however, showed that that originally both languages used both verbs. I argue that English lost weorðan (the cognate of werden) when it abandoned the bounded construal of narratives, inherited from Germanic, while German further grammaticalized this system, with the further grammaticalization of werden as a consequence.
The original variation is illustrated for Old High German in (2), where sein functions as actional passive auxiliary (next to the frequent occurrence of werden), a choice no longer available in Middle High German (3). Similarly, Old English (4) still has passive weorðan (next to be), but this verb has disappeared from Middle English (5).

(1) Latin Ecce aperti sunt ei caeli
“See: opened are him:DAT heavens” (Mt. 3.16)
(2) Old High German Senu tho aroffonota warun imo himila
“See, then opened were him:DAT heavens” (c830)
(3) Middle High German Und secht die himel wurden im auf getan
“And see the heavens got him:DAT open made” (c1466)
(4) Old English & him wurdon þærrihte heofenas ontynede
“and him:DAT got immediately heavens opened” (c1025)
(5) Middle English and lo! heuenes weren openyd to hym (c1384)

I argue that Germanic was a moderately bounded language. Bounded languages construe situations as completed sub-events, emphasizing narrative progress, and make abundant use of time adverbials (Carroll, Stutterheim & Nuese 2004), which split up an event chronologically and often take up the first position in a verb-second system.
In German the bounded system became further grammaticalized, for instance through the fixation of the verb-second system. The concomitant grammaticalization of werden is explained by its bounding change-of-state semantics that denotes completed events. In English the bounded system disappears, as can be seen from the heavy decrease of time adverbials of narrative progress (þærrihte ‘immediately’ in (4), but mostly þa ‘then’: Kemenade & Los 2006) and the confusion of verb-second-syntax (Los 2009). Weorðan, being highly entrenched in these constructions, disappears with them. By appealing to the bounded-unbounded distinction, it is thus possible to account for a major difference in the auxiliary system between English and German.

References
Carroll, Mary, Christiane von Stutterheim & Ralf Nuese. 2004. The language and thought debate: A psycholinguistic approach. In Thomas Pechmann and Christopher Habel (eds.), Multidisciplinary approaches to language production (Trends in Linguistics: Studies and Monographs 157), 183-218. Berlin: Mouton de Gruyter.
Kemenade, Ans van & Bettelou Los. 2006. Discourse adverbs and clausal syntax in Old and Middle English. In Ans van Kemenade & Bettelou Los (eds.), The Handbook of the History of English, 224–48. Oxford: Blackwell.
Los, Bettelou. 2009. The consequences of the loss of verb-second in English: Information structure and syntax in interaction. English Language and Linguistics 13(1), 97-125.
Zieglschmid, A. J. Friedrich. 1931. Werdan and wesan with the passive in various Germanic languages. Germanic Review 6(4). 389-396.