Why did the Boeing 787 switch back to separate IRS systems? The Boeing 737 uses two independent IRS systems (see also Why are there two IRS switches on the 737NG?):


(source: 737 NG FCOMv2 (11.10.21 - Flight Management, Navigation - Controls and Indicators))

and the Boeing 747 uses three independent IRS systems:


(source: 747-400 FCOMv2 (11.10.9 - Flight Management, Navigation - Controls and Indicators))

For the Boeing 777, they switched to a single ADIRU:


(source: 777 FCOMv2 (11.10.10 - Flight Management, Navigation - Controls and Indicators))

The reasons are explained in this answer to the question How many Inertial Reference Systems are on a commercial aircraft?:


  Fast forward to the Boeing 777, the pilots only have one such unit: the ADIRU, or Air Data Inertial Reference Unit. The ADIRU by itself is a fault tolerance unit - within this unit, there are multiple gyroscopes and accelerometers. In the past, when multiple redundant units are provided to the pilots, there were instances where the pilots had trouble identifying the malfunctioning unit or switched to the wrong unit, and the plane crashed. Now there is only one unit, and the redundancy mechanism is auto-managed.


This makes perfect sense to me and should have improved safety. But I was surprised to find that the Boeing 787, which is in part based on the 777, switched back to two separate systems:


(cropped from source)

Question: Why did they change back? Was there something wrong with the 777 ADIRU?
There were also other practical end use issues as well. Having worked in the manufacturing and repair of theses fault tolerant units, one of the benefits was supposed to be the ability to plan maintenance. You could have a gyro or acceleromter voted out and still be able to take off and fly, since the F-T ADIRU had six gyros/accels and the SAARU four gyros/accels. Problem was the airlines weren't really taking advantage of this. They'd wait until you'd have two gyros or accelerometers voted out before and then be unable to take off due to restrictions. Some of the sensors sat voted out for years, with no replacement occurring.
Of course the airlines pointed the blame back the design. The F-T ADIRU was the third most expensive part of the 777 behind each engine. Most of them also did not carry spares for their repair inventory due to the costs of spares. Spare F-T ADIRU a couple years ago were running north of 2 million a piece. They found that the aircraft had less down time with a split system then using the fault tolerant system. Largely because they had ample spares for the split systems due the spare parts costing 1/10th the cost of the fault tolerants.