Alchemy appears to threaten paper Magic
If Alchemy does become the primary format for Magic Arena in terms of design, support, and player-base then it will eventually ruin the paper formats as many of their cards and metas will no longer by insync. They'll practically be two different games.
This also sets a precedence to allow Wizards to change ANY card which could invalidate older paper collections. That said, what incentive is there to continue puchasing paper magic anymore?