Except Apple doesn’t have a monopoly, it doesn’t come anywhere close to having a monopoly. A quick web search for Android phones proves that.
The argument was that Microsoft had a monopoly on the web because they included Internet Explorer with Windows.
Apple has much worse today with WebKit. It wasn't actually all that big a deal that IE was preinstalled with Windows - it continues to be the case and most people just install Chrome or Firefox ASAP.
iOS + App Store + WebKit is a much worse situation. You have the illusion of freedom of downloading Chrome or Firefox for iOS via the App Store, but Apple dictates that there can be no alternatives to WebKit. This gives Apple immense control not just over the App Store, but over the internet. People setting up a website can't just ignore iOS/WebKit - they have to actually make it work. And since Apple sucks at implementing standards or documenting WebKit, it means that to see if it works, you need to buy a pile of Apple devices, because WebKit isn't available for Windows. It's not good enough to just buy iOS devices for testing - you'll see that they don't work, but you won't have access to a debugger. You have to go and buy a Mac just to debug your website. Safari/WebKit are the new IE - entire offices of developers have Macs, not because of any actual competitive advantage, but because it's required for them to support WebKit. Chrome/Firefox/Edge might add new features that developers want to use, but they can't, because Apple won't support them in WebKit, which means the feature can never be on iOS.
Besides all that, you end up with major security issues like last week where Apple had to push out an emergency patch for WebKit, and everyone had to install it. Imagine if Apple hadn't caught that. Literally every browser for iOS was susceptible to the issue, because literally every browser was forced to use WebKit. On macOS, it wasn't a big deal - just switch to Firefox, Chrome, or Edge (is Opera still an option). This is just an enormous security risk because of this policy.
These policies don't exist because it's beneficial to the end user or to developers or anyone else. It's nothing but a monopoly. Apple controls a large segment of the market, and they use that to impose anticompetitive tactics that strengthen their position without actually having to make a superior product.
Having a theoretical competitor doesn't make you a monopoly. You can't say "Android" competes with anything Apple makes - Android is an OS, and I can't swap from Android to iOS or vice versa. The iPhone competes with a whole variety of other phones and devices - nobody is arguing that Apple has a monopoly with the iPhone. The argument is that Apple has a monopoly with the App Store and with WebKit on iOS. If either relaxes, the entire thing falls apart. Apple will fight tooth and nail to make sure it doesn't change - an alternative browser would mean people could have more functional web apps and not need the app store. Another app store would end the need for the iOS App Store. Apple can't compete, and so instead they just make a rule saying that they won't compete (which, the fact they have the market clout to do that means they have a monopoly.)