drop away
Gradually become less
To drop away is to diminish gradually or fall off: Attendance dropped away after the first few weeks.
It can describe physical separation—something that literally falls or recedes: The cliff dropped away beneath us.
The phrase often suggests a slow, quiet disappearance rather than sudden departure: One by one, her supporters dropped away.
deteriorate
Grow worse over time
wane · ebb · weaken · atrophy · decay · degrade · slump · fail · collapse · sag
vanish
Cease to be visible
peter out · trail off · dissolve · evaporate · recede · withdraw · retreat · fall behind · lose ground · thin out