How can subtilisin still function without its catalytic triad?
If the substrate still binds to the protein, other parts of the mechanism are still in place. Specifically, the main chain amide groups that help to stabilize the tetrahedral intermediate are still present. In the absence of serine, water will attack the carbonyl. In the presence of serine and the absence of the histidine, serine will still be the nucleophile, but a less efficient one.

Look up oxyanion hole if this part of the mechanism is unfamiliar to you.