For Marx, since investment is part of aggregate demand, and the stimulus for investment is profitability, accumulation will continue unhindered as far as profitability is high. However, Marx saw that profitability had a tendency to fall, which would lead to a crisis in which insufficient investment generates an insufficiency of demand and a glut of markets.
Marx believed that investment as a component of total demand and motivated by profit would sustain accumulation so long as profits remain substantial. Nonetheless, Marx recognized that profitability tends to diminish, resulting in crises where insufficient investment leads to inadequate demand and surplus markets.