Wizards of the Coast has recently announced that the new Modern format will replace Extended in the upcoming Pro Tour: Philadelphia, and will likely continue to do so at other major events.
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As the format grows in popularity, expect key Legacy-legal cards to drop in value if they remain on the Modern banned list.
Perhaps the most egregious examples are Jace, the Mindsculptor and Stoneforge Mystic, which dominated the Standard environment (often combined in the same deck) for several months until their eventual banning this past June (ending a nearly six-year span of no banned cards in Standard). Both are still legal in Legacy, but banned in Standard and Modern. As a direct result, both have seen a major reduction in value on the secondary market.
The current ban list for Modern: