If the debt is cheaper as compared to equity, why not all debt? Think about the businesses which are very stable, even though they are having equity.

You are right. However finance manager job is to keep the cost of capital minimum. As you said that a firm should opt for suitable debt and equity structure, but what for this structure is required if we can get away with a cheaper source which is debt (more easy to arrange). According to theory that debt is risky as bank or debt holder can take over your asset in time when you will be unable to pay which sounds right. Nowadays when the banks have excess fund, they can restructure your debt as they did it for Dubai world. Isn't it that company failed to take right candidates for a specific job through recruitment consultant (useless), and these candidates lack skills of further research and following the same old ratio system. I think there is plenty of space for development in the field of finance.