specifically in my thinking at the moment is an ADIRU
there are 2 aboard 777. plus the unit itself is fault tolerant.
What we are forgetting is that the crew were experienced
This in itself is meaningless, history of aircraft accidents is full of examples of where an 'experienced' crew essentially contributed to the catastrophe.
The only plausible theories
No, there are many other 'plausible' theories, some many of us would never think of. Aircraft were brought down by highly 'implausible' causes.