There just needs to be incentives to stay around even when a key won’t time.
They could add participation currency to the end of dungeon chest in the form of cosmetic or mount currency that increases with the key level.
I’m a fan of just putting myth track gear into the chest on very high keys (15 or a 20), but anything is better than what’s current.
Also for leavers who don’t leave a party but toon swap and troll with the “offline” trick, the system should remove them from party automatically if they login to a different toon.