What's your move in this position?
You can't just imagine all companies as perfect frictionless spherical interchangeable cows. In reality companies as large as Amazon can have significant market distorting effect, while much smaller companies might have to choose between their principles and going out of business.
What about taking out an ad to let people know where to buy your cheaper item? People didn't know walmart.com existed until a tv ad told them. Doesn't seem unreasonable to take some of the saving from not selling on Amazon and use it for services the platform was providing (discoverability).
Nothing changes until it changes.
What I'm describing is already what happens, by the way. Look at Anker for an example.
Which is why your scenario here wont happen. Thanks for clarifying so well why your argument "there is no consumer harm" is bullshit, what you said here means you know that what Amazon does here increases prices.
Prices then rise back up as your nearest competitor was smart enough to not lower price below their cost.
Or do you somehow believe that price competition currently only exists everywhere EXCEPT on Amazon's platform?