I think they should double their offer and keep the duration adjustable:
Be one week exclusive, get 100% for two weeks,
Be one month exclusive, get 100% for two months.
Then, it becomes a game for the publisher. Launch on Epic and if there are no transactions, abort after one day. But if there are sales, hold on a bit to pocket the 100%.
Publishers will gamble and stay on Epic a bit longer to get better percentages from the strongest fans but they have to end exclusivity to capture the entire market.