I recently left Amazon, and I don't think the stack ranking is to blame for the reduction in talent (which I also noticed).
Amazon doesn't pay top dollar for engineers, and the delta between Amazon and other companies is growing every year. (The compensation ends up being competitive when you take into account stock growth, but the new hire offers are not attractive.) And it's a very results-driven, stressful work environment. The effect of that is that people who get better offers go elsewhere, both new hires and existing employees. Those who are left either don't feel like interviewing or interviewed but didn't like their other options.
Amazon doesn't pay top dollar for engineers, and the delta between Amazon and other companies is growing every year. (The compensation ends up being competitive when you take into account stock growth, but the new hire offers are not attractive.) And it's a very results-driven, stressful work environment. The effect of that is that people who get better offers go elsewhere, both new hires and existing employees. Those who are left either don't feel like interviewing or interviewed but didn't like their other options.