Even if their employment is a positive ROI for the company (which is obviously nearly impossible to even guess at for most ICs at a company the size of Amazon),

Even if their employment is a positive ROI for the company (which is obviously nearly impossible to even guess at for most ICs at a company the size of Amazon), doesn't mean they can meaningfully improve (or diminish) that ROI by modulating their amount of effort. Sure, we can contrive unrealistic scenarios like doing literally nothing for years while collecting a huge salary or lucking into finding and fixing a trillion dollar bug, but for most BigCo workers in most positions? BigCo financials will be BigCo's financials, even if you work every weekend and skip every little league game. You are a passenger on the BigCo bus, not the driver.