Browsers are huge attack vectors as they allow you to run arbitrary code. I wouldn't say there isn't a case to not allow third parties to run a platform in your platform.
They just don't want competition on their platform, and Apple is typically remain small enough to get away with that position. Particularly in stronger pro-anti-trust places like the EU, where iOS is less than 30% market share.
Besides, nothing stops you from still running arbitrary code in iOS downloaded from the web. You're simply restricted to doing that either interpreted or via Apple's JIT. In either case you're still getting arbitrary execution that Apple never statically analyzed, interacting with any APIs that the app wants that code to have access to (intentionally or not).
In fact, I'd argue that it reduces security, since users can't install more secure browsers than Apple's own one (and no one would care to even make one since it won't be usable).
If you have an evidence-based argument to make that Apple has a monopoly in the market for mobile operating systems and has illegally used it to try to obtain a monopoly in the market for web browsers, I would be intrigued to hear it.
(in other words, not every bundled web browser is automatically a violation of antitrust law; nor is every attempt to forbid something on iOS automatically a violation)
I don't see it as a valid excuse. Apple are clearly damaging the industry and market with their behavior. By blocking major features in their browser and banning all other browsers, they are forcing others into supporting their specific requirements, because otherwise they won't be able to reach iOS users. Examples like trying to sabotage DASH for video streaming by not supporting MSE on iOS are quite obvious.
Anti-trust was intended to prevent such kind of outcomes. But it was diluted to the point of being practically useless, allowing the likes of Apple to run amok with their monopolistic garbage.
Microsoft did things that actually broke the law. Nobody's yet convinced a court that Apple has done things that broke the law. And Apple is demonstrably not doing the thing Microsoft got in trouble for.
So you're free to argue that in your ideal world, if you made all the laws, Apple would be put out of business. But you're not free to argue that that's what the actual laws in the real world say to do.
> in your ideal world, if you made all the laws, Apple would be put out of business.
In the ideal world with functional anti-trust, they'd be held accountable for their disgusting anti-competitive behavior and they'd fix it. In the current world their monopolistic abuse runs unchecked, causing damage to the industry and progress.