I'm beating my head against the wall here, but I think I just have to come back to your entire point being if they have money in the bank, the money they have spent and, more importantly, will spend on Xbox is not an opportunity cost. I just have to forever differ here. The money in the bank is not important. It doesn't exist, for the purposes of evaluating Xbox. The entire decision point is if whatever they're spending on Xbox could be spent better. The money they're not spending is not at all relevant. (On that note, if they had today's information in 1999, they might have said "you know, actually leaving that money in the bank at negative growth considering inflation actually would be more profitable than this venture." If they're thinking of the next 15 years, who's to say that evaluation changes.)
In reality, Microsoft has multiple multiples of decision points on what to do, and Xbox has continually made that cut, as has frankly just keeping some capital on the sidelines. However, it is not inconceivable that investors that want Microsoft's money out of consumer devices and into further enterprise development could win out.