>>happening in Amazon too
It was in Cory's article about Amazon that he invented the term "enshittification." In his opinion, for the reasons you have mentioned here and elsewhere, Amazon is the canonical case of enshittification, though he thinks Apple is not far behind.
He has particular hatred for Amazon and Apple because they use their monopoly power to push prices up by forcing deals where people cannot charge less than they charge on those platforms and then enforcing roughly a 30% commission when it is all tallied up, that means that even a business that sells on their own site has to sell for 30% more.
I always wondered why Amazon didn't just let me buy a Kindle book on the iPhone app. Now I know: they would have to raise prices by at least 30% to people who want to buy through the app.
In addition, those commissions mean that any product that only has a 25% margin can't even be brought to market in the first place.
And, as you have highlighted, since you're stuck with the marketplace, most people who sell on Amazon are forced to advertise on Amazon.
Anyway, I could go on telling you things you already know, but yes, not only is it happening in Amazon too, Amazon is so bad it is responsible for the very idea of enshittification.