I never understood why 40% is too little to be considered free and clear from monopoly laws. Why not have a smooth gradient

I can't imagine how you would implement such a scheme. For example, forcing Apple to allow non-Safari browsers in the app store is a binary choice, not something you can implement on a gradient. On top of that, market share tends to move around, sometimes a lot, so you would be changing the rules of the game constantly.

40% definitely is low enough to escape scrutiny as a monopoly, in any case. More than half of everyone who buys a smartphone chooses something non-Apple.

So if every top vendor did that, you'd have something like a cartel, except it wouldn't just be high prices, it would be "once you're in our ecosystem, no more interchangeable parts for you, we own the whole stack". It's a bit like feudalism!