Is your point that Curry is good? It makes sense that Curry off the floor will make the team worse.
This stat has nothing to do with Curry being good or not.
This shows when against opponents’ non-starters, with combinations of Klay, Draymond, Iguodala, etc + various bench players over 5 seasons, we have failed to beat opponents’ bench all 5 seasons.
Including 14-15 bench which you incorrectly think was great.
When Curry wasn’t on, the 14-15 bench (with combinations of Klay Draymond Iguodala Livingston) loses by 3.2 points per 100 possessions, against opponents’ bench too.