I expect the reason there is a legacy playlist is more so they can herd the (presumably small and getting persistently smaller) Vanilla adherents into a decently populated area. Otherwise most of the modes and strike playlists would be ghost towns. Ever try to run the lesser modes from BF4's China Rising once the newer DLCs came out? You couldn't even find a server. I expect Bungie are just trying to avoid that.
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True but they could have gone with the highest currently available Playlist instead of dropping it down to 20 and making it useless.
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I guess so, but to be fair, it's all useless now. Xur has sold every single exotic in the base game by now-- most, twice or more. The only things you're not really able to get now are the Roc, Dragon, and Tiger specific weapons, right? And they already nerfed the only useful one of those.