Emirates are remarkably exceptional, they even offer a daily B77W to NCL. So having Emirates doesn't really mean there's scope for that much more long haul IMHO. And there's nothing wrong with being dominated by Ryanair, STN has way more European destinations than LHR, on modern and efficent Boeings.
I can't see MAN losing their MAN-SIN service of 30+ years just to open up another London airport for SIN. It's not like the longstanding MAN-SIN traffic is going to use STN.