I wrote my undergrad thesis on the origins of Yiddish.
Without getting into a linguistic discussion about creolization and the influence of older Jewish languages (specifically Loez (Old Jewish-French/Italian)) (and the fact that I don’t have easy access to any of the sources I used for that paper), in short, “ים” is a plural marker that is attached almost entirely across the board to Hebrew loan words and may be a Hebrew substrate attached to the German base of the language. It’s not necessarily a corrpution of -en in German.
That aside, as many have pointed out here, living languages change. That’s how you ended up with 26 dialects of Yiddish and my very Litvish grandmother OBM not understanding her friend’s Yiddish that was from a different region (IIRC somewhere in Austria/Hungary, i.e, not especially mutually intelligble).
And, as others have pointed out, as long as it doesn’t affect how you read Hebrew when davening / leining (which is completely different and should remain that way), does it really matter that much?