You're making the assumption that companies need to accomplish a constant amount of work, and that they only hire enough developer to perform this work. I would argue that many companies have a limited budget, and will continue hiring developers to expend this budget. Once hired, there is effectively indefinite work that can be done, and which will better the position of the company. If each developer can do 5x more, then the company will accomplish 5x more, not spend 5x less.