This is where iphones are a risk actually. Android webapps on old phones with their original OS installed have a better chance of working compared to old iphones with their original iOS version. This is both due to Apple's continued stubborness in keeping Safari releases tied to iOS releases (eg. "to get the banana you need the gorilla holding the banana and the entire rainforest the gorilla lives in...") and the dominance of Chromium browsers in webapp testing.

On old Android phones it's easier to install newer browsers without having to update the OS.

Edit: your comment is also not valid on occasion. I've recently witnessed some banking mobile webapps being broken for long periods of time and one bank that decided to remove the agreement approval function from their webapp, forcing you to download the app in order to approve updated agreements.