I think you have to recognise that Manston isn't a charity and Infratil isn't a donor. It's supposed to be a business, and the only reason Infratil bought it was to make money. If it isn't making money and doesn't look like making money they will inevitably close it, regardless of what the airport fan club say.
For me the question is just one of timing. On land adjacent to the airport, the local Council is proposing a huge housing development. This signals the end of the area as a collection of coastal towns and the beginning of a coastal city, as they concrete over the agricultural land in the centre of the peninsula.
Manston is a valuable piece of real estate. If it fails as an airport, the owners can legitimately propose building on it. With the Council themselves proposing massive development for the area, they will be unable to resist a parallel development proposal for the airport. It is difficult to imagine Infratil making more money from it as an airport than they could make by developing it as a brownfield site. The only question for me is when they throw in the towel and go for the honey-pot.