I think one important thing is going unsaid. 約束された勝利の剣 doesn't
mean Excalibur. 約束された勝利の剣
means The Sword of Promised Victory. It quite literally translates to that, and that's all the phrase
means. The significance of the phrase 約束された勝利の剣 is that Fate/Stay Night has developed it to mean Excalibur all on its own - that is, this phrase doesn't mean Excalibur outside of the anime.
Japanese Excalibur page
Page for 約束された勝利の剣 (which doesn't exist)
Yes it's just Wikipedia sure, but if 約束された勝利の剣 was a real term, then you bet Wikipedia would at least have a page on it. Additionally, the エクスカリバー page makes no mention of 約束された勝利の剣 anywhere.
If the Japanese want to say Excalibur, then they write エクスカリバー, because it's a loan word. This makes 約束された勝利の剣 Fate/Stay Night's own term, and in Fate/Stay Night, Excalibur isn't the "established" pronunciation of the term, it *is* the pronunciation of the term. Cultural habit has nothing to do with it. If this was a phrase used outside of the anime to generally mean Excalibur, then I would agree with your reasoning much more Hellacia, but it isn't. In this case, the context of the anime really is everything because the anime has created this phrase.
I hope this makes sense.