Back in the good old days, when we had a full-up set of distinguishable case endings, we didn't need prepositions, but as the case endings started conflating we needed them to make ourselves understood.

Yes, I've heard this, too. Only problem (I have with it) is that even heavily inflected languages have pre- or postpositions (governing certain cases). Also, it might be a chicken or an egg thang: do case endings fall into disuse because folks are using fixed word order and prepositions, or do they start using fixed word order and prepositions because of their case endings going away (because of some phonological changes).