Reading comments here about a new service ex man and STN by Hainan, suggest that if STN doesn't work the man route could be jeopardised in some way. It's important to remember that the airline will operate on a commercial basis, Ie if STN doesn't work, MAG won't want to lose the route ex MAN etc etc. the airline will be in a strong position opposite mag.
It seems perfectly logical that mag would want to develop STN and if an incentive including MAN can be the vehicle then why not.
Lack of connectivity at stn with non low cost carriers makes it challenging for long haul carriers. Equally looking at BA's recent foray into london chengdu shows how difficult it is to develop and sustain new long haul routes, and that from a key hub. BA cite other challenges such as visas etc as factors. So it's not straight forward by any means.