I don’t think HAL have ever sold it as an eco charge.

It started as a “we need to recover our Covid losses” fee. And an attempt to divert people to use the car parks (it’s essentially the same price as 30 minutes) to declutter the airport forecourt.

But whilst running HEX through the night might seem a good idea it would also involve Paddington Station having to stay open at some considerable cost to Network Rail not only for security but paying signallers and other staff. And the costs that NR would charge HAL may not outweigh the benefits.