God why is flight all full this weekend?

Have ex-TPE ticket, D class. Departing Saturday 31st to SYD. Problem first leg, TPE-HKD is waitlisted still, others have cleared. All cabin are full. Can't exercise Diamond's J-cabin ggurantee cuz that is only for J/C fare and not D fare. Also the flight I wanted is in the morning (within 24hour deadline) and ticket purchased this morning for tmr departure. If the first leg don't clear this all falls apart.

Question: Can I exercise my Economy class gurantee to force myself into Economy cabin using "D" class ticket for a later flight outside 24hou limit as my stop-loss to ensure first segment is used and flown, so at least I have confirmed economy on first leg, and waitlist for D-class till departure for earlier flight

Any thoughts? MPC thinks not possible and refused. i'm calling another agent.