This assumes that promotions depend on effort, which is more false than many people believe. Usually promotions are based on business need. The company has a

This assumes that promotions depend on effort, which is more false than many people believe. Usually promotions are based on business need. The company has a high-profile initiative; they need someone to lead it; they pick from a number of qualified candidates for the job. That person gets promoted, either beforehand based on the role they're filling, or afterwards based on their accomplishments delivering the initiative. If the company is fast-growing there are lots of such opportunities; if it is slow-growing there may be none. If there is no opportunity, there is no sense in working hard. What would you even be doing? Oftentimes people who do a lot of work for the sake of doing work are negatively productive, because change for change's sake creates externalities that then create lots of other work for coworkers while making the company as a whole miss out on real market opportunities. Your optimal strategy for one job vs. many jobs depends a lot on the concrete opportunities available to you. If you happen to land a job at a fast-grower, you will probably be so busy that you don't have time for other jobs. If you are at a company that is basically steady-state and just needs a body to occupy a chair, you might as well occupy many chairs.