Any attempts to "run down" or close LHR as suggested in the report (or SEN, MSE, LCY or STN for that matter) would certainly be very expensive and probably illegal.
I can accept close, as was first suggested, but with all the problems you mention. I just don't get how this has now been watered down to "reduce", as some kind of acceptance that the FBI argument is nonsense.
How exactly would you pay BAA/FV to "tone down" their business operations? How exactly do you calculate the net present value of fees lost by halving their movements?
Then you have the nonsense idea that BA might accept a reduction in volumes there too, or perhaps even have 3 hubs (LHR, LGW & STN)? On top of this, you'd have the reverse nimby "close fights over my back yard" argument between the two approach paths, resulting in another legal battle over who is affected most, and almost certainly a legal fudge which actually kept both runways open.
Also, how exactly do you redevelop half an airport site? This becomes even more of a fantasy than FBI itself. So either LHR stays open, in which case the case for FBI is completely undermined by its lack of anchor tennant and increased distance from key passengers' homes/offices, or LHR is forced to close through some lengthy legal process. At least by the latter option you get some "cash back" on the development site, but not the £10bn+ Silver has previously suggested, as LHR is already an extremely compact airport form, worth far more open than it would be closed.
Meanwhile, TfL talk (correctly) of there being little by way of a model for a twin hub, but then fail to mention that opening new airports tends to leave the original one open (KUL, YUL, NRT, PVG, ICN, TPE, CDG and so on...............) far more often than it closes it (HKG, DEN, MUC, JED still waiting on BER). In fact, as we all know too well, in Montreal, it was the NEW airport (Mirabel) that closed, not Trudeau.