DL also has relatively few flights at SFO, so if ATC comes down and says airlines need to reduce the schedule by another 10%, for Delta that's less than 10 flights, so the odds of it hitting you are slight. Also I'd bet that DL wouldn't hit your SEA flight as they are are trying to connect people TPAC there, and especially a flight from SFO, means they "stole" someone from *A. Much more likely that they'd cancel a flight to MSP or SLC or DTW where it's easy to re-route the affected passengers through another hub.