The rate of profit is bound to fall in any industry which has free competition (as competing firms bid prices down), but it would only fall to zero in a totally stagnant or "evenly rotating" economy: one where the same exchanges/etc are done over and over again due to a totally fixed value scale, time preference, level of tech, amount of resources, etc. Most economies are not like this, least of all in the developed world, so profits are bound to fluctuate both in the short and long run.