In the magical fairy land of Economics 101 a lower headcount reduces costs, and because both their and their competitors' costs are reduced market forces drive the price of their products down by an equivalent amount. That leaves people with more spending power. Which leads to either more consumption of existing products and services, or creation of new types of products or services. Either way making those will employ additional people

Of course in reality this does not work like that for large parts of the economy. But even if it was true companies woulnd't gain anything from AI beyond the ability to be briefly more competitive by being ahead of the curve in AI adoption