I thought at the start of this thread the poster bemoaned increases at "OTHER" London airports not exactly loss of market share to Paris Frankfurt Amsterdam?
Depends what share and of what market.
As you well know the bleed to AMS/CDG/ZRH/DXB/DOH/AUH is not short haul low cost one
The risk of that loss is long haul connectivity, almost none of which is at the other London airports for reasons gone over ad nauseum.
Great local news like Cathay adding LGW or Emirates going all A388 is not driving a proper strategic benefit in that area. This was exactly why the Airports Commisions saw LHR as the only viable growth area in that market sector.
Choosing LGW, a second choice if you will, revisits the mistakes of the 70s and 80s with some airlines ending up at LGW only because they can't get into LHR. As that puts them at a competitive disadvantage against their competitors at LHR, the business case means the asset, the aircraft can be deployed via a Euro hub where their partners are stronger and offer support and feed, all of which LGW lacks. BA for example, are not looking to promote LGW as a hub, mainly because they cannot make it pay on that model.
Look at China Southern's AMS presence versus their LHR one. Garuda have become the latest airline in those markets to walk away from LGW, following Vietnam.
Different airports have different places in the wider startegy of the business.