Personally, I think Amazon's original advantage was definitely in a high-quality experience. (FWIW, I've been a customer since 1997, but didn't join Prime until 2010.) They used to have pretty reliable reviews and real products. They used to have better prices. And they used to have a much better user experience.
What I really want is shopping as a utility. I want to spend the minimum time possible and get great results with no worries. Speedy delivery is sometimes part of that, but not always. If there were something as reliable as the Amazon of 2010 but defaulted to 3-day delivery for free, I'd switch in a heartbeat.
I trust the interface much less. Just this week I had to send back two things that were mistakenly ordered. One was clothing. I set the size I wanted, then clicked on the color I wanted. The size I wanted wasn't available, so it switched the size without telling me. The other was a tool. I was searching for Dremel sanding accessories. Mixed in with Dremel-specific stuff were things that weren't compatible, but I didn't notice the switch.
It is now packed with ads of various forms. I want them to be on my side, showing me the best stuff. But instead, they are getting paid to show me stuff without regard to quality or usefulness.
The mixing in of stuff from others stores is another area where my trust has declined. If it were a separate site, that would be fine. But when I want to just buy something from Amazon, I now have to evaluate a bunch of possibly-dubious vendors on the basis of too-little information.
The reports of counterfeit products and comingled inventory has left me much more skeptical when buying things there.
The reports of terrible warehouse work conditions mean that I trust them less to take care of the people doing the work.
The way many people are now making bank doing store arbitrage, where they buy things at Target, Walmart, Trader Joes, etc, and sell it at high markups means I trust the pricing much less. I also suspect Amazon of marking things up more now than they used to.
And finally, their experiments with various shipping options means less reliable delivery. If something comes via UPS or Fedex, I know when it will arrive and trust the drivers will put it in the right place. But their various other shippers provide a different and usually worse experience.
When they started, it was magic. They took all the confusion and stress out of mail order. Now they've slowly been putting it back in. I'm sure it has increased the revenue metrics of various sub-sub-teams. But they've taken me from being a loyal customer to one ready to switch.
I'm with the other person saying much of the same thing. Far as less reliable, they might mean all the people and companies paid for fake reviews. Google for that to find a lot of reasons for not trusting Amazon results.
Another thing Amazon is doing is competing with companies selling product through them. Competitors might differentiate a bit by making a deal, contract and/or charter, that they will never make products that compete with those using them as an intermediary. They'll always be a facilitator and nothing else far as those kind of contracts are concerned. That could help in that current and long-term risk might be reduced.
What I really want is shopping as a utility. I want to spend the minimum time possible and get great results with no worries. Speedy delivery is sometimes part of that, but not always. If there were something as reliable as the Amazon of 2010 but defaulted to 3-day delivery for free, I'd switch in a heartbeat.