Sancho you say above that- "I'm not convinced, by the way, that 'affordable' rents will be substantially higher than target rents in the majority of the country"
If that holds then the affordable rent regime wont create the extra funds to develop new social housing, which is after all its remit. Hence if it doesnt then no new developments will materialise and the affordable homes programme will have failed to achieve what it set out to do. Rents will still have risen and so will cost more to the public purse in housing benefit.
The alternate view is that AR rents will be considerably higher than LHA rates (which as a national average ranges between 54 and 69% depending on what figures are used for gross market rent) and this will impact massively in the public purse bill.
Either way and with all the possibilities between these two points rents will increase and so will the public purse bill of housing benefit. AR therefore cannot be irrelevant as its least possible influence will still place upward pressure on rent and be realised