ORIGINAL: HarryC

Because it is already descending. In steady state, all forces balance each other. To commence the descent an acceleration is required, so downward forces must exceed upward forces. Ignore other peripheral forces and assume that the plane is kept in a level attitude and that power is reduced so that it slows down. Less lift occurs so a vertical acceleration commences. As long as lift is less than weight the downward acceleration will continue forever with the rate of descent trying to increase to infinity (or it would try to until air drag or solid ground or the limiting speed of light gets in the way). A constant rate of descent is no acceleration, so the upward and downward forces are back in balance. The descent continues as described by newton's laws of motion, an object will continue on its path until an unequal force disturbs it and causes an acceleration.