Businesses don't expand just because capital is cheap, they need some kind of demand to fill. That's the fundamental problem with "trickle down" economics, it assumes the economy is supply constrained when it is more commonly demand constrained. And worse, the polices that it produces tend to squeeze the lower and middle classes, further reducing demand.
I was of the understanding that most orthodox schools of economics fundamentally view demand as infinite. Supply-siders hope to meet demand by increasing supply, lowering prices, and making more goods available to those with fewer resources. Policy-wise, this is achieved through production subsidies, fewer regulations, and tax cuts.
Their opponents (demand-siders?) hope to increase access to goods by increasing the purchasing power of consumers. Policies like social safety nets, public education, and tax increases on the investor class promote this goal.