No, Ian W, you have missed the point. If an aircraft id fitted with Phase 3 screens in all positions, then if the problematic RF occurs, all DUs will be lost - they all have the same design and thus the same susceptibility. Probabilities of simultaneous failure under normal circumstances are remote in the extreme, but in this instance it's as if all were run from the same source or you just had one large DU taking up the whole panel - one goes and the lot will go as all are subject to the same RF.
It would not seem unreasonable to have an AD that directs airlines to ensure only one or two Phase 3 DUs are fitted per aircraft, allowingt hem to swap units between ac on their fleets to minimise the potential of the issue while having little cost implication, allowing the faulty units to contiue in use as long as they are outnumbered by other Phase units. I think we are all able to cope with that.
It is amazing that so many airline bosses publicly decry the need for the AD and its fix while being the first to shout at a press conference that safety is "their number one priority". But these bosses shouldn't need to get so wound up because the manufacturers should be footing the bills. Surely the sales of goods acts (UK term, but must be applicable equivalents globally) would require this? It would be repugnant if the customers (pax and airlines alike) have to pay for the faulty parts to be replaced.
These DUs are not a luxury, and while the loss or one or two of them is little more than inconvenient, the loss of all would be a major issue. Whoever said we could use automatics through the MCP to fly on sby instruments is incorrect - with no DUs, how do you know the ADIRU and DEU data is correct? That is what is going to the automatics. How do you know an AFDS mode has engaged rather than been armed with no FMA? Sby instrument flight is strictly hand flying.
Some corrective action is certainly required for some aircraft, and CEO reactions have been cavalier to say the least, but it does seem the AD is a little poorly thought out.