at what point does this render the pistol unfireable?
AS far as I know, never.
Gas cutting is a self-limiting process it starts, reaches a point & then stops permanently. Like the other poster I had a DW, but in .357 S&W Magnum, & it showed gas cutting almost immediately. Once it stopped (20+ years ago) it just did absolutely nothing at all no matter how many thousand rounds I fired.
Allan Quatermain: “Automatic rifles. Who in God's name has automatic rifles”?
Elderly Hunter: “That's dashed unsporting. Probably Belgium.”