What is the aim of the football club and why do you support them? Surely it isn't to make money, it is to win things. If it existed purely to make money then we're doing a terrible job of it and if that was the main aim I think I'd stop following football altogether.
Money obviously plays a big part for a lot of owners but winning things is still more important to the fans. I know a few Man Utd fans and they are far less happy now just being a rich team than they were when they were winning things.
I think they're two different questions. I support them because I love the sport and they are/were my local team. Obviously there's an emotional connection and it's brilliant to win things but I think if you look at it objectively, the target of any club/business is a commercial one - to grow commercial over time?
The promotion/cup question is interesting as it gets at the heart of the matter. From a "sporting glory" perspective a cup is a bigger achievement but from a commercial perspective it is dwarfed by the money promotion would bring (which, if invested sensibly, could set the club up for sustained commercial + sporting growth over time).
I agree with your point about commercial growth btw, it is incredibly difficult to make money in the sport but for me that's got to be the ultimate objective, as in many instances, commercial growth enables sporting success (obviously by no means a guarantee but it can help massively)
A cup win, whilst obviously to be welcomed, is unlikely to change much long term (although we'd have European football which may enhance the standing of the club enabling us to shop in a better market for players), whereas promotion could, if managed correctly, be a significant change in the long-term prospects for the club.
The Man Utd example, I think, is more about how that money is being spent rather than the fact they are a rich club in and of itself. Success should have absolutely have followed with the investment they've had, the reason for their frustration is mismanagement of their situation.
