I don't see why Amazon would do any of this. They want to destroy competitors, rule the market, and make as much money as possible. Exclusivity helps with that, books selling better because they're in KU helps with that. Unless you're JK Rowling or some other household name, you aren't "big" to Amazon and they don't care if you're in KU. There are thousands of other authors for people to read. It's like Netflix. Have a few big movies and shows, fill up the rest with a boatload of content.
As for stopping scammers, when Amazon removed a bunch of scammers before, what happened to the rate? Right, nothing. Because the KU pot is nothing to them.
As far as I can see, exclusivity doesn't help with that. There may have been a time when it did, but, as I pointed out, competitors' market shares shrank for other reasons. The only competitors Amazon really knocked out with exclusivity were the small independent ebook stores which I suspect would be gone by now anyway. The major players when KU started are all still around.
It helps by draining the other stores of indie authors. Just because Amazon's in the lead doesn't mean they're aren't going to try to dominate even more.
I have no doubt Amazon would like to dominate as much as it could. However, there are practical limits. It's improbable that the Trump administration is going to go on a major antitrust binge, but the US could conceivably change adminstrations as soon as January 2021. All that is needed is for the Justice Department to drop the Reagan era interpretations that led to not invoking the law very often unless the consolidation caused consumer prices to increase. Going back to the original view of the law could leave Amazon in trouble as it is. That could be a strong motivation not to increase its share of the US book market beyond what it already is.
Back in August of 1997, a lot of people were shocked when Microsoft bailed out Apple with some much-needed cash. What Microsoft got in return (dropping a lawsuit Apple probably wasn't going to win anyway) was minor. Why did Microsoft make this deal? Because Microsoft, already under antitrust scrutiny, wanted to keep Apple from collapse so that the Mac operating system could be pointed to as a viable competitor when the Justice Department came calling.
In some ways, Amazon is in a similar position. It doesn't dominate every area of business that it's entered, but in certain areas (online book sales come to mind), it might be vulnerable to antitrust action. Amazon has far more to lose by having some of its practices declared illegal or by being broken up than it has to gain by driving its bookstore "competitors" out of the market. Even if they gained a modest increase in market share, (which is all they'd get even if all indies went wide) that wouldn't really matter to Amazon, for whom books now constitute only a small share of the business.
It's also worth noting that Amazon can't really destroy its major competitors even if it wanted to. Aside from Barnes and Noble, which has been teetering for years, the other will be around and probably stay in books as long as they can make anything at all with them. Google, Apple, and Rakuten (owner of Kobo) are all major corporations that aren't going anywhere. That means there's an upper limit beyond which Amazon can't really expand its business.
It's still possible that Amazon is thinking in exactly the way that you describe, but if so, it's taking a very skewed view of the future. It would arguably make more money if it dropped exclusivity and got some big, wide indies into KU. Crushing what's left of the competition isn't practical and might actually result in far greater losses down the road.