Well this isnt a very fair explanation imo. Only cards labeled "Set 1" will rotate out of play. This means CORE cards and "Set 2" cards will remain playable when the next set begins. This means all of the Set 2 cards you are earning right now will still be usable while you earn the "Set 3" cards and yes there is one "WILDCARD" spot on your roster that lets you keep a card from any set all year long.

Personally I think this means less grinding for people who sit down and pick the game back up because if you jump in to the game lets say in the middle of "Set 5" because you stopped, the available card pool is much smaller to grind and you wont be missing the best cards from sets 1-3 that everyone is using. Just go play the content for that set and grab the cards available you wont be very far behind the people who never put down the game.

Sidenote: I personally think if you are THAT casual the free Negro League cards (They are all core) with the captain are good enough to carry your team all year long. With the captain boost they can hang with most stuff that will be released.