SIA basically has shot itself in the foot. It was poor foresight on their part and good foresight on Cathay's part. Check this article out from 2011. John Slosar predicted then that having too many brands would be an issue for SIA. He also empahsized that the focus should be on whats happening in the market and not on establishing a bunch of companies...
Cathay explains why it will not follow other Asian carriers in launching an LCC subsidiary or brand | CAPA - Centre for Aviation
Now SIA is introducing premium economy to boost yields but with Scoot serving almost the same destinations as SQ I'd say Scoot is eating into SQ profits and at the same time Air Asia and gulf carriers are also giving it competition.