> 9 out of 10 people i ask this is the single reason they stay at Amazon.

9 out of 10 I ask say it's about reliable delivery (not necessarily next day). Most people order things because they need them and have read the reviews, not because they are speculating at something they may need or are ok with getting something else. Clothing being a notable exception, unpredictably sized and variable quality, which nobody is happy about.