Extending the line to support a particular development is different. House prices in London are very dependent on proximity to a tube station. The developers of Battersea Power Station and Nine Elms will make considerable profit as the extra value they can sell the apartments for will be greater than the cost of the extended line and station. This is more comparable to the construction of the Heathrow Express or the Heathrow Crossrail link.
Upgrading TFL's trains on an existing line, which makes TFL a profit, of which only some of the passengers are going to LHR isn't the same.