The focus on price, alone, is a distraction. The monopolist can choose winners and losers, favour or exclude sellers or buyers, unilaterally determine and dictate dispute resolution (e.g., Amazon's purchase dispute process), in favour of buyers, or sellers, or parties specified on other bases.
Or as comms providers have done in the past, and it is feared they might do if network neutrality obligations are lifted, as the FCC are attempting to ram through presently.
The monopolist isn't answerable to the public or citizenry, as a government body is.
A monopoly market is not a competitive market, with many buyers and sellers, to which any given buyer or seller has a recourse should one counterparty decline business or terms.
A monopolist creates a situation of rents, by controlling and regulating supply. This allows for price inflation if demand can then be influenced, much as is the case with land.
That's just off the top of my head. Any further questions?
But my argument is that there must be provable consumer harm, and that consumers are the only thing that matters.
So all those bad things that you mentioned, do they hurt consumers AND is Amazon actually doing them? That is the only thing that matters, and would be the thing that should be determined by whatever court case happens.
There may be highly probable harms which cannot be directly proven, or for which various standards of proof are thwarted by the monopolist itself. Since "wealth is power" (Thomas Hobbes, Adam Smith), monopoly power itself conveys additional power. There's a strong argument for additional responsbility, limits, and/or oversight as a result, for which there's a long list of supporting argument (Smith, Mill, Marx, Galbraith, off the top of my head).
Secondly, consumers are only one of several parties potentially affected. The other groups may be competitors, suppliers, vendors, the public at large, natural systems, etc. I'd have to think over this at greater length.
The argument that price and "consumers" are the only factors of significance in considering monopoly harms is a distinctly modern one, promulgated almost exclusively by monopolists themselves. To rather great effect.
"If prices forever stay low, then consumers ALWAYS benefit, right?"
Not necessarily. The system is much more complex than simply "low prices = good".
Markets are an ecosystem, if you allow one retailer to dominate, they will exert control. That could be by hiking prices, or it could be by picking favorites among their suppliers, deciding which products are "suitable" and so on, and by squeezing suppliers, possibly to the point where the result is stagnating quality and innovation, as suppliers race to the bottom on price alone.
Quite apart from consumer political issues of having such a dominant presence in the market. Eg, if they refuse to serve you for whatever reason (or serve you badly, perhaps with "custom" pricing), you are disadvantaged.
The consumer ultimately loses in this long game, and it should go without saying that enabling a monopolist, even one with low prices and supposedly a focus on "customer satisfaction" (though is that merely a vector for dominance?), is likely to end badly.
If prices forever stay low, then consumers ALWAYS benefit, right?
You are making a different argument than the one that the article makes, FYI.