Is worship stemming from fear or from love, taking into account that the absence of reward and punishment may reduce the number of worshippers?

Fear is a motivator for worship, but not the only one. Love is the head of the bird, and fear and hope are its two wings. The scarcity of believers does not negate that fear is an impetus for worship. A Muslim should worship their Lord out of love, hoping for His reward and fearing His punishment, without these assumptions.