Dies to Removal

This is true, but it is nevertheless a barrier of entry into Legacy (just not as big as Modern due to ABUR duals). The reprint could give an incentive to the same people getting into Modern to look into Legacy, albeit at a much lower rate.

Their use in Legacy is a significant factor as well.

I anticipate these new fetches will settle down to $10-12 a couple of months after release. Like RTR and Theros, this will be the most opened set in Magic history, and the market is going to be flooded with these cards. The only thing that could change this is if there is no other real value in the set. If the other rares and mythics in the set are underwhelming, it could be a disincentive for shops and casuals to crack boxes for value.

Zen fetches will see a significant decrease (and already have) for a couple of reasons. One is that a lot of modern decks are playing off-color fetches and can just as easily replace them with the new fetches. For instance, I play two Misty Rainforest in my UR Delver deck, but someone wanting to get in modern and play the deck can simply swap the Misty for one of the blue fetches in KTK. This will reduce the demand on the Zen fetches. The second reason is aprehension in the market for the
possibility that the Zen fetches will be reprinted in the third set in Khans block. This is a completely reasonable fear given what we've been told about the structure of the block (time travel perhaps changing the plane) as well as WOTC's recent policy on wanting to provide a 10-land cycle in each block.

While Zen fetches will certainly lose their value, the overall prices of decks will not. The value should move away from the mana base and into the staples like Snapcaster Mage, Cryptic Command, Noble Heirarch, etc due to increased interest in the format. If you've been on the fence about buying in to these staples, now is your time to commit.