I don't think this is the right solution for activities, especially for BRs. Dropping the minimum cop requirement to 0 won't make BRs more enjoyable. Bringing a BR with 0 cops online at 4 AM still won't make much of a difference, because even if there are no cops at first, they can simply log in. By the time the BR reaches the first or second door, three cops can easily be online, and with 8–10 defenders under the current door timings, there is almost no chance for the BR to succeed.
I think it should stay at a minimum of 5 active cops, but with a few adjustments to the door system. Make the doors open faster when there are only 5 cops online, while keeping the current scaling where the more cops there are, the faster the doors open. Right now, when there are only 5 cops, the doors are just way too slow.