And five, six cases isn't near enough to obviate prepositions. You'd need probably twenty or more.

Yes, I don't think there are any languages which have been studied on this planet which don't have prepositions. The number of cases in PIE has not been determined, IIRC, or their number has not been agreed upon. There are languages which do not have grammatical cases.

Ceci n'est pas un seing.