As I pointed out elsewhere in this thread, Iron Banner and Trials require active, dedicated support and development resources. Since legacy consoles will not be generating any revenue, paying for those resources has to come from somewhere else. This is not a profitable, sound business decision, which is one of the major reasons for the upcoming changes. Also, Microsoft has already said they're discontinuing support for the 360 this year, and Sony probably isn't far behind.
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Make them not require active support.
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Changing them to not require active support means a large amount of development, and I suspect additional hard drive space that the game would need. Lack of hard drive space is one of the reasons RoI is not being released on legacy consoles.