The 21st century aviation market is comprised of distinct segments, Skipness, and those segments behave according to their own specific patterns. Hub transfer traffic is just one of these segments. LGW's core markets are leisure and short-haul business travel, primarily serving those resident in Southern England and overseas customers visiting London. In serving these markets, LGW has already proved itself by evolving to become the busiest single-runway commercial airport in the world. That stat alone tells you that LGW is no Mirabel! And over coming decades, London and the South will require further capacity to satisfy these two growing market segments for which LGW has amply demonstrated its suitability.
The desire of some LHR advocates to focus solely on hub transfer business is akin to demanding butter but denying the need to provide bread. The market segments which represent LGW's core business are growing, and demand for them must be met somewhere. LGW is the obvious choice for that job, and barring an economic depression (entirely possible, but would impact all sectors) the airport can fill a second runway given the timeframes we are discussing in this process. Leisure travel growth may be less glamorous than hub transfer traffic, but London's airports system must accommodate it just the same.
Of course, the LGW expansion price-tag still needs to make economic sense too. That is not a given, and any (strictly limited) public funding requirement must be comprehensively scrutinised to ensure taxpayer value for money before being approved. No blank cheque must be proferred by the treasury. Any LGW redevelopment must be delivered overwhelmingly by private funding (as promised by GIP).
But to deny that there is a need to provide for growing demand from the less-glamorous O&D leisure / short-haul business sectors in which LGW specialises implies shielding one's gaze from an inconvenient truth.