The modern take on stock investment is to not give dividends, so the only way for shareholders to make money is to have the company grow indefinitely.
Obviously a capitalist won’t tell you that. My economy professor kept insisting that efficiency is always positive because it only concerns making a bigger cake, so there is more cake to be divided among the people involved, which he called surplus. In reality greater efficiency has a cost, and the cost is paid by people, while other people pockets the surplus. Fuck capitalists.
Your original point was that the pie doesn’t need to keep expanding, so you have been disproved I would say. Saying that the economy need to keep expanding is the same as saying the economy is based on infinite growth. If it stops expanding the stocks are not profitable anymore and the shareholders are going to vote to replace CEOs, so the CEOs have an incentive to make he companies grow at any cost.