Why can an industry enter the decline stage of the industry lifecycle model?

Question

Quizplus · Accepted Answer

The Answer is The industry can enter the decline stage due to declining sales because of 3 key reasons:
Changes in demographics: Toward the end of the baby boom in the 1960s, demand for baby food dropped and rivalry among the leading firms-Gerber, Heinz, and Beech-Nut-intensified considerably.
Shifting consumer tastes and needs: Social trends and health considerations have resulted in declining demand for cigarettes and tobacco products since the 1980s.
Technological substitution: Word processing software led to the decline of the typewriter industry; online streaming and downloading of movies is replacing DVDs, which replaced VHS cassette tapes as the medium of choice for movies. Sales of DVD players, discs, and movie rentals are therefore declining.

Why Can an Industry Enter the Decline Stage of the Industry