Im assuming the new modes Will be released the instant the dlc is released. Horde mode with objectives/small boss fights might be super fun (a la Mass effect 3 online mode). You're basically Being overly pessimistic.
A raid we already know is 20hours of gameplay for first clear and 1hour Each week after that. Im Almost Certain the two new modes will easily beat that.
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Trials of Osiris is an event so I highly doubt that it'll be out day one. They need to keep us playing and there isn't enough content, even with HoW to do that till the next dlc. And how about instead of bringing modes that have been done before and are being worn out, they make something unique and new, like they promised the game would be.