There comes a point when airport expansion is no longer a benefit, and instead becomes a burden. Manchester reached that point some time ago, and passengers have begun to suffer. Eventually, they get so frustrated with the long walks, queues, and types of inconvenience cited above that they take their business elsewhere.
There are some good things about Manchester Airport. If you go to train stations in places like Darlington, Blackpool or Huddersfield, you will see 'Manchester Airport' on the departure screens. The direct train services, though eye-wateringly expensive, are a boon.
Secondly, in some cases it is possible to transfer airside, thus avoiding those horrible security people.
Thirdly, err... that's about it for good points. Ah no, the Aviation Viewing Park is actually quite cool, and the cafe does good chips.
But Manchester's exponential growth in recent years has brought additional destinations and frequencies at the expense of convenience, comfort and hassle-free travel. Some are willing to pay that price; others are not.