I completely agree with you.
I guess I'm just wondering why the expansion and infrastructure of Manston seems to be a more ludicrous idea (or at least that's my impression of how it's viewed) than Boris island considering the locations aren't too far away.
Personally in terms of specifically SE development I'd favour a Gatwick runway, better utilisation of Stansted with improved infrastructure and then in third place another runway at LHR.
Realistically I would bet money LHR will get another runway just out of sheer convenience.