Amazon has robust competitors in the shopping space (hello Walmart). And does not often serve niche markets. Hobby electronics (think old radio shack or current adafruit), sewing, wood screws in volume are all places where amazon falls flat and there is plenty of room for others to succeed.
What amazon is doing is brilliant, it isn't monopolistic, it is vertical integration. Something that doesn't happen as much as it once did.
Amazon owns 49% of US online retail. 
If 49% still allows for robust competitors, at what % does that change?
Amazon has a leg up on Walmart for online sales, but Walmart is still the number one retailer in the U.S.
This complaint isn't about the shopping space, it's about the selling space. Small businesses who want to sell online will typically get a whole lot of nowhere if they don't use Amazon.