Compare this to Backpack that was also discontinued but since it's paid for monthly, 37signals decided to let existing customers continue to pay and receive their service.
So Backpack stays alive (albeit without new signups) since it continues to make them money, but Breeze gets closed and all customers get shafted. Even thought Breeze was sold as "forever" while Backpack was sold as "monthly".
And let's not forget that Backpack needs a lot more (security) updates and maintenance than a simple mail server for mailing lists.