Original sin is an explanation of why salvation is necessary, not a historic even.  It can no longer exist under the Eden myth on one end and the punishing God myth on the other, but teachings like that can change.  The Resurrection must be a fact of history for the Church or there is no Church.  The Assumption is a tradition that has been co-opted by those who believe that the Immaculate Conception was a necessary part of the salvation story due to original sin.  It can be believed in without the baggage.