Standard QF response campdoag? Nothing else I post on PPRUNE is normally 'standard Qantas response'- and a reading of many of my previous posts would tend to confirm that I have no qualms with sledging QF if it's deserved- so I'm not going to start with this issue.
However, an 'appeal to authority' and the fact that SQ, CX, EK, etc are all flying doesn't do much for me. I'm sure we've all got examples whereby other airlines did things that we all found strange- you quoted one in your post. SQ attempted to take off in Taipei one night when others were saying 'no' and look how that turned out. I've watched SQ (and others) disappear into massive CBs ex SIN too but that doesn't mean that it's 'safe' to fly.
Perhaps other airlines have more info that QF have- wouldn't be the first time. Based on my reading of the issues though- and not just based upon what QF tell me- I can only say that the prospect of flying under an ash area of unknown/forecast base and unknown/ forecast concentration doesn't fill me with the warm fuzzy feeling that I like to have whenever I go flying.
A couple of other points. The density has direct relevance if you fly into it. If you're basing your flight on flying at FL230 because the models say the ash is all above FL300 then that's your call. The question is though, how do we treat forecasts of turbulence? With scepticism? Do we think that because the forecast says that the turbulence is FL280 to FL350 that we can't get 'bounced' at FL250 (or below) or above FL350 due to the same 'system' that is causing the forecast area from 280-350?
For me, I'm glad that QF made the decision not to fly. It saved me from having to do a LOT more work to ensure that the decision made by office whallahs wasn't going to impact on the person who actually has the legal responsibility for the flight- IE, me.