FCT might be excellent, but I would rather spend 13 hours in the air on SQ enjoying the in-air experience rather than flying on LH and wishing I'd changed the flight to SQ

Besides, if you fly SEA (F) FRA (C) LHR (F) SIN, you get to use the FCL at FRA which I feel is a small enough sacrifice for the much better inflight experience on the long, long flight.

Isn't the LH board divided over whether it's possible to visit the FCT when flying into FRA in F and continuing in C? You could potentially do the LHR transit and achieve everything you wanted, including the FCT access!