But you also won't get promoted just because of seniority, most of what people say about being hard to get promoted is true. However, there are only 3 SDE levels if you don't count Principal, so SDE II has a very large range of pay rates, and you don't need to get promoted to get a higher pay. Higher levels are all about being a better leader and influencing more people. There are people who spend 10 years as SDE II and are happy with it, because they don't care about leading others, they just care about doing their job, and this is fine.