I just paid to enter a Historic Event queue, and my first match was against a [[Field of the Dead]] deck, clearly this opponent had joined their Event long ago, and is now winning against players who cannot be reasonably expected to prepare for a paid play mode featuring banned cards.

Here's my suggested fix:

• When cards become banned, any player enrolled in a paid Event using a deck with now-banned cards gets matched only with other players in that category.
• After a grace period, as the available pairings dry up, simply award the 7-win prize and drop them from the event.

Those prizes are not extravagant, the number of rewarded players would be tiny, and it would be far preferable to kerbstomping invested players who pay for events more often.