"There’s nowhere to go. Amazon has either destroyed or bought every competitor that has ever come close to its retail business."
Without this, nothing else matters. If there are alternatives, then you keep using Amazon and Prime only if they're worth it. If Amazon ruins them, go somewhere else.
Personally, I think it's completely wrong. I buy on Amazon because it's extremely convenient and because I can be pretty sure that they're going to offer a good price, likely the cheapest price available.
However, any time I make a purchase that's more than $30 or so, I shop around. There is no shortage of other places to buy anything I've looked for. They usually have higher prices or worse service than Amazon and so I skip them. Occasionally they're better, and then I buy from them.
There's no lock-in, there's no monopoly, there's nothing that makes people stick with Amazon besides better prices, better service, and inertia.
If you don't like it, go somewhere else. It pains me to say this because that phrase is so overused, but it applies to Amazon well.