Blizzard has the option to do this permanently, this can be changed to other expansions too.
What’s the benefit for the player with a weekly lockout?
Just move any legacy content to a daily rotation 
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I don’t have an opinion one way or another, but I think their intention is to keep rare collectibles rare. With ReMoP, people will (easily?) be able to get otherwise rare drops in MoP content. This gives people an alternative to farm on their mains instead, if they’re not interested in the remix.
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Probably for the same reason daily quests suck. This is a bit of an apples vs oranges thing but personally I think the weekly lockout is fine.