Where in the USA makes a lot of difference. A UX vs. hub-to-hub flight makes a huge difference in the probability of delays or cancellations.

68 minutes is a plenty of time for a D->I connection. April is the tail end of the rainy season at SFO so big weather-related delays are a concern, but the chance that the 12 minute difference between your original 80-minute connection and the new 68-minute connection being relevant in missing or making your connection is exceptionally small. And even if it is, there are enough SFO->TYO flights that the chances of you being stuck for very long if it does happen are also quite slim.

Since you seem a bit stressed by it then I might choose to connect in LAX anyway if you want a more relaxing trip. I wouldn't concern myself too much with the possibility of the LAX flight being canceled. It's a very real possibility that they'll remove it from the schedule, but if you have a removal/schedule change it'll probably be 30+ days out, and United has enough alternative flights that some acceptable option will be available somewhere.