It is about how much BA makes, because BA makes enough money to pay the pensions. It was not a "promise" like some people like to call it, it was a legal document, a contract of employment, and they can't just change things they way they want and anytime they want.
No, it was not - you don't help the cause by posting incorrect facts. A defined benefit pension is not a promise - what you mean is that they have a moral obligation to pay.
The trustees could indeed change it unilaterally - not that anyone in their right mind would do so, hence all the action to convine all and sundry that they cannot pay. The fact that it is in the contract is irrelevant quite frankly - legally there is no obligation.
Pure and simple - this is a moral obligation around which people have planned their retirement, and upon which basis as part of the package, they recruited huge numbers, and for which they can (probably) afford to pay through some means.