I think you mean a big haircut for the later stage investors. Even if Uber has a down round, the earliest investors will probably still have positive returns (at least on paper).
You are confusing a down round with a small exit event (here and in other places in this thread). Liquidation preferences don't come into effect until a sale of the company - so no one is "crammed out" by them in a down round.
Though these two types of events are obviously correlated (a down round makes a smaller exit more likely) they are not the same and should not be confused. A financing at a valuation that is a billion dollars less than the last round would be a down round, but a sale of the company at that same valuation would still far exceed the liquidation overhang.
