In the '00s, you did used to be able to fly cheaper if you were flexible enough to not care about a specific flight time.
It was called Priceline bidding. But that faded away and was fully discontinued a few years ago. What OP suggests is effectively an extension of this, where the airline can choose alternate dates as well as times, and doesn't have to assign you a flight at transaction time, but can assign you a flight later. If the consumer demand wasn't there for the original proposal, I doubt it's there for the much greater uncertainty that OP has.