Unlike iOS where basic features like the web browser require system updates, Android is modular and updates get pushed through other channels independently of the OS itself.
I'm not sure what you mean by not worrying about iOS needing OS updates for browser upgrades as the Safari engine is the only web browser engine allowed on iOS and third party browsers like Chrome are just skins on top of Safari and not real counterparts to their desktop cousins. Upgrading the OS is the only way to get new web functionality and bug fixes.
Web browsers have to use the Safari engine but that doesn't mean they don't also separately update the browsers and add functionality. I.e. you mostly get modularity by not using Safari even if you have to rely on the underlying Safari engine being updated. Most of the features users notice are updated by the app provider anyway.
I'm not sure if you're being sarcastic here--all iOS browsers are custom skins on top of Safari.
Yes I'm aware of WKWebView and how it's not the same as Safari. I'm using the classic meaning of browser skins, dating back to browsers like Maxthon which were wrappers over Trident in exactly the same way.
No I'm not being sarcastic - I think calling the apps a skin on top of Safari is a bit of an over-simplification. Most features that users notice happen at this skin layer, not the rendering layer. It's peak HN to really be caring about this especially as Apple offers (as far as I know) no performance difference across browsers using Webkit or w/e. You could actually just say that Safari itself is a skin too, just the default one that comes with iOS.
It's similar to complaining about other basic features of iOS IMO (like complaining there's a default settings app or that iOS just works a certain way).
What about security updates or new HTML features? Chrome or Firefox on Android get security updates for many years after official system updates end. The same is not true for Apple.
Don't think most users know or care about security or HTML features so while certainly it's a difference it's unlikely to be important for most users.
Think of Safari as a skin that's unbundled from the engine. While Chrome or Firefox are reliant on Apple to update the engine, so is Safari, but neither are reliant on Apple for other functionality that they want to implement that users can take advantage of.