Please consider adding some sort of Exchange market for the cards, not simply dissolve. 

Every long-time player has plenty of duplicate cards and no more upgrades available. Not the only option is just to dissolve the cards. And with Exchange market it would be possible to exchange excessive cards for some other cards from previous events for example (not mythic of course). It could be different exchange rates for different cards (for example: 4-1 for the same value, 8-1 for adjacent, 16-1 via one, 32-1 Common-Leg) . Or this exchange rate may depend on the age of the event. The card for exchange may be  changed daily or weekly. 

This will allow players to upgrade old events cards and to stay playing game for longer. At this point the old events card are useless unless one max-out them during the event.