Well, Old English baggs to differ. English lost its case markings on articles early on and kept them on nouns a while longer while German kept them on articles and simplified nouns much more early on.

That sē is still the determiner, now with an additional function as an article, not an independent article. What I said applies to the article as its own thing, i.e. when "the" and "that" were already independent words - in fact their decoupling is directly tied to the same loss of the endings that caused the morphological case system to go kaboom.

Again, German didn’t dump anything into articles but rather lost it everywhere else.

I'm talking about the informational load, you're talking about the phonetic changes.

There is this idea that this fostered the process of using der/die/das much more often (which made it from a demonstrative to an article) but I disagree because it was a widespread process, not only in German but in huge parts of Europe, including beside Romance languages also English were this reasoning doesn’t work (as shown above).

It's actually both a shift promoted by interactions between languages in the Western European Sprachbund and the result of simple sound changes. Much like a vicious cycle:

• noun endings get slightly muddier due to syncretism →
• people rely more on a default word order to convey case →
• higher usage of demonstratives as "poor man's article" (definiteness might not be the same as topic, but in a pinch it's close enough) →
• poor man's article becomes an actual article →
• there's less pressure to keep the noun endings distinct, thus against sound changes that would merge them →
• noun endings get slightly muddier due to syncretism

Higher usage of demonstratives as articles might be also caused by interference of other languages - that guy spamming "that" and "one" in a language will eventually do the same if speaking some another nearby language. And it also explains roughly why German ended as the exception, as it's right in the middle of the way between "case endings, no articles" Polish and "articles, no case endings" Romance.

Then, in German you got that weird middle ground where word order still conveys topic, but the noun endings already weren't conveying the case any more. The info gets dumped in the article - and that prevents further sound changes and regularisation processes from attacking them.