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.
For Marx, investment being a part of total demand and driven by profitability ensures continuous accumulation until profitability remains high. However, Marx noted that profitability often declines, leading to crises characterized by insufficient investment causing demand inadequacies and market surpluses.