HKG - SFO and SIN - HKG are the actually top 10 most profitable routes of SQ. However things have changed a lot for HKG - SFO, when this route used to be one of the most profittable one, however there's a big change.
1. After Cathay Pacific started flying this route (CX is actually the newest operator), competition on this has raised much higher comparing to the old days when only SQ and UA were flying this route.
2. Prices have dropped sharply, thus the profit would drop. Actually fares are always the same for the 3 airlines, usaully UA is the cheapest and SQ is the most expensive. However there's only a very short difference, and that also means that airlines would earn less because the price is actually at the floor, if the 3 airlines are trying to make a decrease in price, they might be seeing red.
3. Business travellers have also dropped. 7-8 years ago when i travelled HKG - SFO (I also travelled HKG - HNL which was even a long time ago) i still remember i nearly had to travel J myself. Why's that? Because the 24 F class seats were FULL. YES, full. And you can't even expect 12 to be full these days. Also at that time the F class and J class tickets are more expensive then nowerdays - Not bluffing, but it's true. Also there wasn't really anything called an alliance in the old days - If people were travelling in J or F, they were surely paying themselves, not getting free upgrades. From the 24 F class wholly occupied comparing with nower days only 4-5 seat occupied out of 12 - Maybe everyone's getting an upgrade and F class tickets are even cheaper than older days, how can you expect an airline to earn that much?
4. People would use the less money to travel to its destination - And transfer flights could do that job. Here in Hong Kong there are lots of travel agencies which would take transfer flights - To save costs and to lower the price of a tour. Obviously great news for JAL or Korean Air, even China Airlines, but not for CX, SQ or UA.
I'd say that the decrease in business travellers could really affect the profit of an airline on a certain route. Sometimes if you sell a first class RT ticket, you can't even gain the same amount profit if you sell 10 Y class tickets. So business travellers could really influence the proft of the airlines. Also increasing landing fees, mainteance, fuels...oh well.
Lest we forget that SFO and TPE have exsisted for a long time...also HNL for a short period of time. To be honest there are lots of airlines in Hong Kong that have 5th traffic rights to fly to another destination from Hong Kong except its origin. I could easily count them out. Just that the situation of SQ is indeed much stranger...i guess SQ is the airline that have the most fifth freedom rights from Hong Kong. Also they actually have rights for HKg - ICN already, the problem is will SQ launch the route. I bet there will be more to come...except ICN. Probably Moscow?