Pace, surely if the manufacturers could come up with what is definitively safe (and I'm not talking about what MO'L was saying earlier), then it would take the argument away from the CAA. What's actually happened during the year since the last ash disruption? Seemingly, not a lot.
As for CB avoidance........slightly easier than avoiding barely perceptible by eye amounts of ash. Furthermore, it's much easier for Met authorities to issue SIGMETs/forecasts for CBs than for ash.