At Google at least, L5 (Senior) is considered a "terminal" level, in that once you reach L5 you can generally settle into your IC duties without fear of being f

At Google at least, L5 (Senior) is considered a "terminal" level, in that once you reach L5 you can generally settle into your IC duties without fear of being fired, as long as you continue to perform at that level. L3-4 you're expected to get promoted every 2-3 years or else eventually get cut. L6+ (generally management levels) is assumed to be only for those who are somewhat ambitious and want additional responsibility. These levels are probably not coincidental: L5 is when you're expected to be able to work independently and have the organizational context to make good decisions and contribute usefully with minimal manager support. Before that level you usually cost more in management overhead than you contribute in output.