It's not an excuse to violate anti-trust. MS were blasted for this, but Apple got away with this foul behavior.
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.