Personally, I don't think it as quite as cut and dried a decision as some.
The BA cabin crew training regarding catastrophic has changed in the last year or so.
One of the first questions they should ask themselves is 'Am I going to die if I stay in the aircraft?'. Yes they must also consider the structural integrity and the status of the flight crew, but the primary consideration is whether or not they will die by waiting a few seconds. If they have to ask themselves whether or not it is catastrophic or not, it probably isn't.
In this instance, by the above definition, it was not catastrophic and a decision not to evacuate would have been valid.
Having said that, I totally understand the decision made and am not criticising it - as always hindsight is easy.
Standing back expecting to be flamed!