@ATC Watcher
A lot of non factual rubbish in your post. How could you say Lufthansa replaced their 747-200F with 777F when the 747's were retired in 2005 and the 777's entered service in 2013? The 747's were former Lufthansa passenger jets converted to freighters.
Lufthansa were initially disappointed with the 747-8 but that is to be expected with every launch customer and every new airliner experiences the same. Improvements were made to the 747-8 and googling will allow you to read about these improvements which has made Lufthansa a happy customer and one that endorses the aircraft freely on the Boeing website. Possibly the A380 experienced more issues than 747-8 when it entered service.
FRA- SIN is not operated in a parallel as you state since Lufthansa has one daily flight. If the 747-8 is operating the route it is most likely due to the fact that the bookings for the A380 are dire since the A380 is the aircraft assigned.
Another fact is Lufthansa cancelled 2 of their A380's on order. How many 747-8's did they cancel?
SQ have never operated the 747-8 and their last 747 passenger aircraft left in 2012.