I can see both sides of the argument.

It would be expensive for DL to allow someone to do change after change to the ticket/PNR after a schedule change or cancellation, and in principle one shouldn't be able to then have a free cancellation option for months until flight time after first being allowed to make a very reasonable change.

OTOH, someone could easily (I think) accept the change on line or especially on the app by accident or without understanding that accepting the change means that you're locked in, even if it wasn't the best option. IIRC you must accept the change, even if it applies only months later to the return portion of the ticket, before being able to check in for any segment. Maybe more importantly, when there's a change that impacts one's trip, there's a natural tendency to panic and book something right away, while there's still ability, even if what you're offered isn't very convenient. Availability can change (especially if the waiver requires the same fare class or payment of fare differences) and even the terms of the waiver (such as the must travel by date) can change significantly.