Arena mostly consists of newer cards, plus a few old ones that they can fit into their simplified rules framework.
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The obvious correct strategy for Wizards is to continue expanding Arena's card pool until it contains all cards. There's no good reason they shouldn't be able to expand it back to Coldsnap in three years. Figuring 25% chance that Wizards does the obviously correct thing and 5% chance they follow some other worse strategy that also involves Panglacial Worm.
@jfjurchen The Arena rules engine is much more restricted than the general Magic rules engine. I doubt it'd be worth the programming time for them to figure out how to make something like Panglacial Wurm work properly.
@IsaacKing Well, I agree it'd be stupid to port just panglacial wurm, but I think there's value in being able to say "we support the entire Modern cardpool" (it's hard to see how they'd be able to fulfil their dream of killing MODO before that) and in making Commander work properly in arena.