Addressing the content you have now added to your argument, MAN does not have the capacity to handle the numbers you put forward because the need to provide it never arose. It seems eminently sensible to me not to throw funds at 'white elephant' terminal capacity and supporting infrastructure in the absence of any requirement for it.
Had MAN gone ahead and provided terminal capacity (as they did runway capacity) based on those wildly optimistic figures they would certainly not be viewed as white elephants today.
The comparison with LGW is relevant as MAN was also forecast to actually overtake Gatwick as early as 2012!
Second runway lets Manchester rival Gatwick | UK news | The Guardian