This is getting tiring, and along with "walled garden", it is really stale and worn out as an argument.
If you could install your own browser on iOS, then browsers wouldn't be a problem. Because of the walled garden, you can't.
"Walled garden" isn't a tired argument against iOS, it is a very serious problem, but fanboys would rather ignore it or call it 'worn out.' In reality it sucks and there's no need for it, and plenty of reasons to not have it.