I think you're underestimating the value of Walmart. Sure for things you can wait to get in 2 day shipping, it's a no brainer compared to driving to Barnes & Noble, or BestBuy. But a large part of the US still shops at brick and mortar stores because they want/need things "right now." Walmart is still a juggernaut, especially in smaller cities, rural areas.
And drone delivery... I don't see that being successful for Amazon for another 10 years. Too many technical, legal, and cultural hurdles for that to happen faster, if ever.