anothertyke's observation is helpful. In fact MAN was already under performing against the White Paper forecast before the financial crisis hit. This wasn't because the forecast was inaccurate, but mainly because MAN's management failed to engage with the leading LCCs, allowing LPL to grow rapidly to over 5mppa. If you looked at MAN and LPL together, the DfT's forecast was pretty good until the financial crisis hit.
So I think it is fair to criticise MAN's former management for some of the shortfall. But, as others have noted, the main players are long gone and are now largely discredited in the industry.
Overall, the shortfall against the White Paper forecast is probably split around 50:50 between MAG's own actions (in being late to the table on LCCs) and the 2008 financial crisis.