Hasn't it been tried more than once, long haul full fare airlines from STN just haven't worked ?.
Whether there is the demand for extra sched flights to Beijing from the south is also debatable.
Looking at the CAA stats LHR-Beijing have trodden water in recent months, up about 1% last month for example.
Add to this Air China chopping the 1st class from its LHR flights , can Hainan make a profit from STN.
If MAG bought STN to build on long haul full fare they may be disappointed ?
Nigel