Before SIA ordered the A340-500, they went through a very detailed analysis of the different competitors, which included the 777 and concluded that this aircraft was its best choice. Therefore, even in the light of its dumping of the -300s, I don't see why it should change its mind. The A340 is unlikely to remain the only FBW for too long, as the A310s will need replacing and the various A330 versions are best suited to this job.
Yes, it's true that the A340 is a bit slower, particularly on the climb, but it is still an excellent aircraft. Airbus's fuselage cross section is ideal and its freight capacity has always been excellent. I have no doubt that the A340-500 will serve SIA very well indeed.