If one considers shorting as a stand-alone strategy, then the expectation of a high positive return over the long-term should probably be fairly muted.

If however one considers short positions as an additional source of capital to be invested on the long side (e.g. 130/30 portfolio), then the potential return drag from these shorts should be compared to the cost of other capital, bearing in mind that the short portfolio does a better job of asset/liability matching (assuming an all equity portfolio) than other sources of leverage.