Average product is not as reliable an indicator of how a business is doing as it used to be because of

That indicator isn't reliable because it doesn't capture the reality of what of a company's performance. A company is judged on quantitative and qualitative grounds. It doesn't tell how much quality is input in the process, how efficient the processes within the company are, how much profit is made, how much resource is spared, etc. It's a flat indicator which doesn't basically provide insights into a firm's operations.