Yeah see, where I don't see eye-to-eye on here is the idea that Windows is bad because it lets you download and run whatever you want. I don't agree at all. I think Windows is just bad, and in fact, it gets worse as they try to push for an Apple-esque App Store model and slowly add roadblocks to """side-loading""" (or as we called it in my youth, "installing software".)
Likewise, I don't think that's what makes iOS good, either. I don't think it ever was. I think that it's understandable that Apple's approach to this would in fact be to make a "walled garden", especially given that it is wildly profitable, but Apple already had beloved computers long before they started doing stuff like this. Yes, to be sure, the Macintosh had and has plenty of flaws, but I do not think that anyone's goal is to turn iOS into Windows, and Apple would retain a visegrip on the iOS experience just due to how locked down modern operating systems are. Immutable image based operating systems with heavy restrictions on what usermode apps can do are going to be inherently much more stable, even when faced with untrusted usermode apps.
The thing is, while I get why I and others get categorized as part of the "anti-Apple" crowd, it's not because I simply hate Apple for no reason whatsoever. Like many others, I desperately wanted to like Apple, and then they just crapped all over what I consider to be my demographic of users. Please understand: to me, I am not THAT loyal to ANY vendor. I am a user of computers first and foremost.
But what's worse is, I have to deal with the knock-on effects of this. My biggest pet issue is absolutely their tirade against patent-unencumbered formats like Opus, which impacts people I know who prefer iPhone because it's all they've ever known. "How can I get WebMs to play in Discord/My web browser/etc?" You own a computer where the manufacturer decided that you're not allowed to install codecs or alternate browser engines, so you can basically go fuck yourself.
"Bicycles of the mind" my ass.
In any case, I think there is no slippery slope here. Apple is not going to volunteer control of their ecosystem away, so god knows they're going to do the minimum necessary to comply with any of this. So I think the impact on the overall iOS ecosystem will probably not be that substantial once all is said and done.
Even on Android, app stores outside of the Play Store are relatively rare, and attempts to circumvent the Play Store for the purposes of making more money haven't done very well. Even on Windows, it's hard to compete with or avoid selling on Steam, because it's got so much momentum and users generally like it as a platform for buying games on. Apple has more than a level playing field, they have a decade head start, total control over the platform, and so much more; They are far from at risk of control slipping.
If they're worried about anything, it is probably quarterly financial reports not being as good as they possibly could be.