Do you have any examples of them making bad UI to get people to use siri?
EDIT: I didn't read the question carefully enough. I don't believe Apple has dumbed down the UI to push people to Siri.
That being said, giving users a choice over the assistant, browser, etc. would be a good thing.
They overloaded the button to load Siri if its held for even a fraction of a second too long. 99 times out of 100 I don't want to use siri but she continually pops up.
Trying to click to get back to the home screen.... here's Siri.
Trying to double click the button, to kill an app..... here comes Siri
So to answer the question....
1) They made Siri enabled by default, something that I can't see myself ever wanting.
2) They made Siri come up using the button, again something that almost always means when siri comes up its by accident and not indented.
Apple isn't immune from "dark patterns" in their UI. The Siri example above being the most egregious example I can think of.
So what's a real example of Apple making the UI worse to further their agenda (i.e. selling more stuff)?
Later, I was able to go to Settings and switch it on or off.
I don't see how your above example is at all egregious -- people retain full control.
Compare this to Windows 10 (at launch) turning on a whole host of privacy-compromising features and not giving clear control to the user (but I was glad to see at a more recent setup I did that I now can toggle it off even at first install, and that I can look it up later)
To steelman your argument, I would propose instead talking about lack of browser rendering engine control or lack of ability to install arbitrary executables. That's actually something people have no/little control over. But even then, there is a reasonable debate to be had.