You are missing investment projects which have been started or completed which assumed a higher price to be profitable. You either mothball those projects writing off very expensive assets or try and minimise losses by pumping as much as you can if the marginal cost is low enough.
Downstream work can't save you from previously investing $1bn in a project which now can't produce the return to pay that back.
Just look at how many companies are going bust in the US at the moment. Whilst the big oil companies won't be going bankrupt they'll have many projects on their books which are bankrupt.
All you need to do is look at the share prices of Shell, Exxon etc.