Personally, I think the traffic flows of the MEB (3 and counting) demonstrate the viability of routes ex MAN. Some of these come off (HKG is a good example) some not yet (eg BKK, TYO).
There is a limit to what can be achieved by the likes of EK at MAN. I know there are plans for a fourth daily flight, but with morning afternoon and evening departures on the largest aircraft on earth it is difficult to see what commercial advantage a fourth daily departure can give.
Into this space comes the likes of SQ (I understand MAN-SIN has exceeded expectations) HU and CX. I very much hope to see SQ get back to daily (with or without the IAH tag) with HU. I also look forward to new carriers entering the market with more flights to Far East hubs.
The other point to make is that MAN does not just serve Manchester. It squarely serves Liverpool as well as Leeds and Sheffield (to a lesser extent other cities). This is a vast market, even by clawing back some leakage from London area airports would make a significant difference.