If ANet wants the big bads to have a failure option, they should give them a reason to go away again rather than just a timer.
For example with Tequatl, go straight to the laser defense on appearance and use the destruction of the laser as the failure rather than the timer.
For Claw and Shatterer the fire bases can be what they are after once provoked.
In essence, ANet should sit down and ask themselves why the various big bads are showing up. Give them a reason to be there beyond being a loot pinata with a timer.
Now if they want to add a timer on top, it should be for a extra prize for impressive performance rather than as a requirement for any reward at all.